Are your front wheels 6 lug? If so you should look for 6 lug rear axle unless you want to carry two different spare tires.
With a TH350 (no overdrive) and stock sized tires, you will want something like a 3.08 to 1 gears if you want to cruise at highway speeds. You won't win any drag races, but you won't have to stop at every gas station either.
Carl
