Of all the possible trans options (40s/50s) , are all 4 speeds granny geared?
AFAIK, that's true. The Chevrolet 3-speed tranny is preferred as it has synchro on 2 and 3, and 1st is geared so low you hardly use it most of the time.
My plan was/is an “original”-ish trans and a rear gear 3.55 conversion. Lots of 55+mph driving in my area.
Ive been told that will work well, I had planned to do that...but now in up to my eyeballs in a T5 conversion, swapping out the torque tube, swapping in a '55 Series 2 axle, and will put 3.38:1 gears in it.
Good luck with your conversion, the r/p are the biggest factor, but to bring down RPM overall on the highway you need OD, IMO. Any pics of your truck?