I am running a set of 3.08:1 gears in the roadster pickup rearend, with turbo350 automatic and stock chev 305 with 4 barrel. My rear tires are 31" diameter. This car is a fun drive around town, and great at highway speeds---but----the roadster pickup is a light little truck, and this rearend gearing definitely does not give the "snap" that you associate with something that looks this much like a hotrod. If I had it to do over, I would look for as set of 3.55:1 gears. Your 39 chev will weigh half as much again as my roadster pickup, so you certainly won't be impressed with a set of 3.08"s.