I've done it this way in the past, choose you engine and trans, find out what rpm it likes to run at, decide the type of driving you will do most, choose your rear tires you need to know how TALL they are to choose the best gear ratio. I had a 62 suburban with a 327 4brl carb, 4 sped trans, 3.73 rear end 33" tall tires and got 17 mpg and it was very drivable.
