Custom truck

A truck tractor would be no different than any other motor vehicle in this regard.
The current procedure with typical automobile restomods is to find a starting point with a car that is pre emission (1975) and here is the important part - with a good title and identifiable vin number, preferably the OEM vin plate. Once you have that you can make any modifications you wish, the only limitations would be what is road legal in the state you are registering it in.
Do exactly the same with a truck tractor, make sure it has a clean title and a vin plate. I am guessing you intend to drive this. If it is just a show truck it may or may not be considered commercial. I would tend to stay with a drive train that is pre 1999 so the ELD thing doesn't get in the way. How you register the truck will depend on the state you are in. If you can get some sort of antique plate for it that may allow you to avoid certain regulatory requirements or a cheaper base plate.
The Antique Truck Historical Society would be a good place for exact information like this.
American Truck Historical Society

 

It depends what your end result is. If you want to build something for shows most people tow them on a low bed or something to the shows.

If you want to drive it to the shows only and not use it commercially most States I think I have some type of a reconstructed title process you have to go through.

To use it commercially depending on which state you're in some are easier and some are much more difficult with getting it certified and the paperwork and title work, although I think that also would be some type of reconstructed title.