Do you know which engine the truck has? Does it need rebuilding as well? Many of the large trucks used a 'Lincoln' based engine. Parts may or may not be hard to find and unexpectedly expensive if you do. Research where to obtain engine rebuild parts before you need them. Also be aware that some 'new replacement' parts are made off-shore and seldom of good quality.
There's a chance using a later version drivetrain (or various parts of one) may turn out to be less trouble / more cost effective in the long run.
Two engine ID links:
http://ford-y-block.com/lincoln.htm
http://ford-y-block.com/Block%20identification.htm
Ford made worthwhile yearly and maybe mid-year upgrades in electrical, carburetion, ignition and mechanical systems between 1955 and 1958 until '63.
Research the changes so you can plan ahead to include these improvements if desired. Some 'later' pieces may be easier to obtain than the earlier ones.
It's your truck, build it the way you want to.
If you are concerned about judged events some judges won't know the difference as long as it looks 'period correct'.
Other shows with pickier judges have various categories for non-original vehicles if it comes to that.
The red & white '55 Bird on the left under my name was missing it's original engine. I debated the troubles of trying to stay original with the engine or go with one a few decades newer. Since I'd never worked on one before I decided 'period correct' would be easier, so I wouldn't have to find or fabricate one-off parts, without a suitable shop & tools.
In hindsight and after discovering the car wasn't as original as I'd been told, I wouldn't mind the driveability improvements of at least a newer transmission.
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