The 1955 first series 1/2 ton could have a standard 3 speed but the truck could also have had the optional heavy duty 3 speed, 4 speed or Hydramatic transmission. The length of the driveshaft (propeller shaft) varies for the type transmission and I don't have these lengths available. It really doesn't matter. Your plans are to modify so you want to measure yourself rather than guess.

https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1345429/1.html
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=935530


"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."