To improve drive ability / top end speed and maintaining period experience. You would be better off keeping your 3 speed tranny and swapping out the 4.11 ring & pinion rear end with a 3.55 ring & pinion. The 3.55 ring & pinion gear set will let you cruise at 65-70 mpg. But you need good brakes.
Check out speedometer in photo, is a says 0 10 20 30 40 50 60 PRAY.
