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Discussion Starter · #1 ·
Looking into building my 63 coupe hardtop into a Pro Street build. Looking for ideas and pictures of related builds on original X frame cars(1958 to 1964 Impalas). Right now the plan is.

1. Re-establish existing bodies inner rockers (rust repair/panel replacement)
2. Remove existing floors from toe board to rear trunk pan.
3. Build rear back half part of chassis for dimensional purposes (lots of welding)
4. Take center axle to center front hub dimensions on old frame
5. Cut old frame at designated points behind the front body mount (measure three times, cut once)
6. Build center section of frame connecting the front old chassis to the new rear back half. This will be a ladder style design with two 2X3 side rails.
7. Now those rockers just rebuilt will need strengthening plates added on the inside.
8. Weld the inner rocker to the center ladder bar frame using 1X1 square tubing between rocker and center frame bars.
9. Build out the back area of the trunk support structure and weld to tail pan.
10. Optional: Stretch rear wheel opening if needed
11. Fit up and weld in rear wheel tubs, make sure to re-establish the rear filler piece (between window and trunk) support structure and trunk hinge support structure if the plan is to keep the trunk hinged and not pinned.
12. Weld in 12 point roll cage, this will act as further frame support
13. Mock up drive terrain so a transmission cross member and drive shaft hoops can be installed.
14. Use steel to create a blast shield around the flexplate and torque converter. (I would like to keep my feet if these parts ever fly apart on a hard trans-brake launch.)
15. Finish out floor with aluminum plating screwed into frame metal.

Now is the time to do any other body work, start to hook up linkages, run brake lines, fuel lines, run wiring, Install lexan front and rear windows, Quarters will probably be lexan also as they will probably no longer be able to roll up and down,....

Looking to see if anyone has done this and if I'm missing a big step or if someone had photographs of a similar process on an X frame car that would be awesome. Currently looking at getting 20" wide tires or more on the back.
 

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You are building a Pro Comp class car and driving it on the street. Only difference is depending upon power levels you might need a parachute and I can tell you that doesn't work out well on the street. Every kid that walks by pulls the pin and now your parachute is blowing in the breeze or wrapped around the axle of the first diesel Power Ram lifted one ton to drive by. If you do need a chute install it on a class 5 XD Reese Receiver style trailer hitch, and don't take it with you to town unless you can guard it.

Weld in two side stringers like was used on all of the other GM cars of the same period. In fact I would start out out with a Pontiac Catalina, Olds 88. or Buick Electra frame and see if I could modify the body to fit the Chevy by installing that car's floor boards and just welding it under the Chevy. Those cars either had a full perimeter frame or an X frame with side rails to box the triangles along the sides of the car for added strength. In other words only the Chevy frame flexed as badly as it does.

I would back half the car using 24", 28", or 32" inch frame rails depending upon how huge you want the rear tire to be. It is the biggest tire you can find that sells the pro street look. Wider the tire the closer the rear frame rails need to be to each other.

I used my 1985 Impala as a Pro Street car because I wanted the full perimeter frame as a foundation and I wanted it to look like an old taxi cab (which it was built to be at the factory as a special order for an Indian who died before he took delivery of the car and no one wanted to buy it so I got a new car for a used car price.). On the plus side an '85 Impala is just a heavy duty suspension Chevelle: as it was the same size and weight as an early 1970's Chevelle, but used full size car suspension parts. It looked like an old taxi but ran 9.39 in the quarter with my 582.

Big Dave
 

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Discussion Starter · #3 ·
Been doing some research and found that the early 59 Oldsmobile used pretty much the same frame, but added a perimeter on each side. I'm looking into this as an option so I can continue to use more of the stock frame, but get rid of the twist. It wouldn't be hard to weld on my own version of the perimeter and be able to position it best to tie in with the roll cage of the car. With the bigger tires and back half design the rear pumpkin should sit high enough that I can use a one piece drive shaft and shouldn't have to worry about drive shaft hoops as the frame would act as such. I'm thinking I should use 2X4 box tubing with a 0.120" wall thickness for the perimeter. Then I can add some 1X1 or 1X2 stringers for needed floor supports and tie points for Seats.
 

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If you can find one with the 1959-'64 axle under it I would rather that rear end than a Ford style nine inch as it is stronger and doesn't waste energy (horsepower) with added parasitic friction due to a low contact point on the ring and pinion.

https://www.hotrod.com/articles/1303sr-1957-1964-pontiac-and-oldsmobile-rearends/

It is also a leaf sprung car so you could run fully adjustable (J-bolt) slapper bars or Cal Tracs and still run in the eights.

Big Dave
 

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Discussion Starter · #5 ·
Probably not going to find a 60's Oldsmobile frame, but when I install the ladder bar rear frame section, it will span from rocker to rocker and intersect the existing frame in the middle. Then all I have to do is weld on the stringers that go along the rocker and the front tie in piece. Just trying to figure out what is going to give me the best strength, a square weld where it kicks in or getting a pre-bent 90 and ending up with 2 welds on either side of it.
 

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As with any structure built for strength you triangulate everything. Two side rails on an X frame changes it from a flimsy twisty frame to rock solid frame laterally; by converting the X into four triangles. To prevent torsional twist you need to build a cage above the frame's plane that consists of a jungle gym of triangles.

Don't forget to box the original open C portions of your X-frame, and to add gussets to the original torque box to make that area stronger. You can cut lightening holes in the added steel to make it look period correct, and bore similar holes in the original frame to insert a tube into that welds into the added steel used to box it to strengthen both pieces by further tying them together (over constraining which was done in the sixties before racers learned engineering theory: and way before tube frames were designed on a computer).

Look at first generation Corvette frames for inspiration. The factory engineers took a full size Chevy frame and cut out 17 inches of the full size X frame and added side rails. They also modified the rear of the axle frame to allow for the use of leaf springs instead of coil springs.

https://www.progressiveautomotive.com/wp-content/uploads/2012/11/Copy_2_of_P9070009.jpg

Pride Auto Plaza of Derby, Kansas - 1961 Corvette - 1961 Corvette

This assumes you are not building a Pro Comp or Super Comp class car for NHRA racing, in which case you do worry a lot about weight so you run a tube frame and 14 point cage from the start. Pro street is all about the look not actually being competitive. This allows you to put all your money into tires that are actually too big and and a chrome plated 8-71 or 10-71 blown motor that is even bigger.

You can run a 12-71 or a 14-71 but not with a distributor, as there is no room for it. Instead a crank trigger and coil on plug technology is required for a 14-71 roots. Have to keep in mind GMC Diesels never grew bigger than an eight cylinder engine so an 8-71 is as big a blower case as was available back in the sixties. Though I doubt if any one could tell a 10-71 from an 8-71 by eye balling it. But your motor can tell the difference.

Just saw your reference to a ladder bar suspension in the rear. That is a track only thing and will bind on the street if turning on pavement that isn't dead level. Have one wheel fall into a pot hole while turning and something will break (or bend in the case of a stock rear end). Better to retain the coil sprung with control arms that uses aluminum with nylon bushed centers than a ladder bar. Better yet convert to a a truck arm suspension which was very popular in the sixties when all GMC pick-ups had them and they where readily available in junk yards. I personally prefer a four link tubular armed with steel hiem joints. Works well on the street or track (whether NASCAR or NHRA).

Big Dave
 
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