Put the transmission in first and see how you like it, going ahead and swappping out the rear gears now may be a waist. Everyone has there opinion of what gears they want to run. The camshaft/engine mods will have an impact for drivability and what you are driving, older cars are not aerodynamic so there is a lot of head wind. running something like an OD trans with 3.08 gears you might think that your gonna get better fuel econmy but 3.73 may actually work better.
Example: I ran across a guy last year who had a 40 coupe with a corvette totally stock LS1, 4l60 OD trans and 3.08 gears and could only acheive 21mpg and he wanted to go with 2.7X gears like the Vette's have. I don't know what his outcome was but I am guessing worse. I run a slightly modded LS1 in my 48 with the 4L60 with 3.73 and get 25mpg.
