Very nice survivor! When a car is this nice, but the drivetrain is not what you want, my rule of thumb is dont change anything you cannot change back by turning a few bolts. With that in mind, the 4 speed is out, because you have to cut the hump. I would put the 350 in using bolt in parts and upgrade the powerglide to a TH350 or 200r4. That should help the acceleration blues. actually, you could put the 200r4 behind the 6 cyl and getting much better accel and GREAT gas milage and not have to change much at all! Remember to save and tag everything you take off for future owners. Have fun!
BTW, power brakes is a good swap, no too much work, But changing the steering is more involved. If you keep the 6, then the man steering should be fine
