Might work ok behind a 700r4. It all depends how you want to use your truck.
I have selected 4.11 gears for my 1940 gmc. I will be running a 302 gmc 6 cylinder and a 5 speed overdrive transmission. With 30 inch tall tires, my truck will do 70mph at 2200rpm.
Without knowing your goals, there is no way to make a reccomendation on gearing. Will you be racing? cruising? hauling? etc.
By the way, there isn't a law that requires folks to put a 350 with an automatic transmission in these trucks. A nearly stock restoration is way cooler than a mismatched chopped hotrod IMHO. Prewar trucks are really rare. This is your truck, you pay the bills and you make the decisions. Just make sure you research so you are happy with the end result. The parts you remove from your truck may also help another truck live.
