You go to the junk yard and find a SBC van. pull the dipstick out and see how it seals (does the end of the tube have a cup to hold an O-ring or is it cut off straight the same size down to the engine. If the tube is uniform in size then you can buy that dip stick.
On your next oil change with the old filter still in place drain the pan. You can pre cut the length of the dip stick down so that it goes all the way into the pan just barely bottoming out. Add two and a half quarts of oil to the pan with a new filter full of oil and then start the car up. Insert your still slightly long dip stick in and pull it back out. With a cold chisel score the dip stick at the oil mark (this is your fill position. Add another quart and do it again to find your full position. Then grind off any excess dip stick below the add line such that it goes in without hitting the bottom of the pan.
This assumes you are using the stock oil pan (advertised capacity of four quarts though it really holds a half quart less) and the half quart capacity AC-Delco PF-25 oil filter. If you are using a "full size" one quart oil filter (5.4 inches tall PF-35 filter) then your oil capacity is the full four quarts.
You really don't want to overfill an oil pan. Especially with a stock unbaffled pan that offers little to no oil control.
An easier solution is to wait until the next car show and with the owners permission pull his dip stick and measure it.