Drop your SBC and 700R4 into it and go driving. Probably no need to worry about rear end gear ratio. My '48 Burb is set up with the 350/700R4 option and it runs great. You didn't mention or I missed it, is the front frame original or sub-framed with a GM clip of a later model? If it is sub-framed you are good to go after inspection of the welds to make sure it was done right and is safe.
