I've all but decided that i'll go with an Eaton Series 3 POSI @3.55. Ugh... now I'm curious if this is all out of my wheelhouse?? Is there anything else i should be doing to the rear end??? Axles? I keep remember seeing something about 30 spline? Also.. maybe a new drive shaft? I swear ive seen something before about stock driveshafts possibly not being able to handle the amount of rotation? But i may have been INTERNETING WAY TOO MUCH.
Its easy to get down a rabbit hole.
If you have as you say a nearly stock numbers matching car then you have the word of the engineers at Chevy who designed the car as you see it to know that everything will hold up just fine. This assumes of course you retain the small rock hard plastic tires of the sixties. Put on a set of large by wide supper sticky jumbo tires then dump the clutch and all bets go out the window.
You can look at the side wall of modern tires and compare Traction rating along with the Wear rating to know how well they will bite for any given size. Almost every new tire will offer more traction than a GoodYear PolyGlass 60 series raised white letter tire, if you could find one.
All 12 bolt rear ends have a 30 spline axle. How good of condition they are in I do hot know. The Spicer design (a copy of the Dana 44 rear end) was designed to use the axle itself as the inner race of the wheel bearing (the bearings rides on the cast iron axle instead of a forged steel inner race that every other rear end manufacture uses). Though the axle is induction hardened by a radio frequency transformer I designed and Jackson Products built it will eventually wear through the hardened surface and go to town on the raw metal of the axle. You will have to pull the axles to install any new differential, so look at them to see if they are still flat or have a groove worn into them by the tool steel roller bearings.
Before you do anything buy this book and read it:
I also recommend this book even though it isn't Chevy specific it does a better job of explaining the procedure to set up a rear end:
I mention this because you only get one shot as setting up a rear end and it requires technique and tools to do it correctly the first time. Once a howler, always a howler.
I used to change and set up a differential once a week because I broke them one a week. This was in my 1953 2 door Chevy sedan with a 409 under the hood. As I got older I got wiser and paid others to do it for me (I actually changed my Chevy rear end out for an Olds rear and never broke it again; instead breaking B-W T-10 transmissions, which I became good at rebuilding). But 12 bolts or Dana 60 rear ends I took to a local shop that specialized in building them (my local Chevrolet dealership). I may have set up a half dozen or so myself but it is too bothersome and I lacked patience.