Richard,
The problem is that if you have a hull in restorable condition, you probably have most of the vehicle. If you were able to find just the hull, then finding all of the associated parts to make it work would cost a small fortune. Yes, the engine, transmission, and transfercase are the same as a GPW but there are so many accessories that just to find them would be a daunting task irrespective of the cost! That is why you must research and understand what is unique to a GPA so that when you are able to find one in restorable condition that you can accurately evaluate whether the vehicle is complete and if the asking price is fair.
To give you an

, the GPW has the same engine, trans and axles, but this is what you need to find to make it work in a GPA:
Engine: the GPA engine needs a double pulley, different generator mount, different starter (12V), different generator (12V), different regulator (12V), 6-bladed fan, capstan pulley assembly and drive lines, etc.
Transmission is the same but has a special drain hookup on the bottom (could use 90 degree street els to serve same purpose)
Transfercase: need special front assembly to be able to seal hull to transfercase, need GPA PTO, need propellor rear bearing assembly, need bilge pump including special brackets
Axles: need GPA springs (more heavy duty and have different attachment than GPW), need to move mounting brackets on axle (at different location than GPW); need rear axle seal assembly (on axle), need rear axle seal tube (from body to rear axle), need vent tube hose that attach to body, need brake hoses that attach to hull; need special seals and assemblies on front knuckles.
The list goes on and on and on. If you started with a hull and then bought all the necessary parts that you would have to restore anyways, I could see easily paying over 100K just to get the parts you needed. Many of these parts are complicated and almost not reproduceable unless you have an original part to start with. Yes, some of us have extra parts but we are very leary to let these go as there is no parts house that we can run to to get the part if it breaks on us.
I have seen some of the original drawings and they are terrible. They (Ford and the Military) were doing things fast and the drawings are not what you would expect to get today.
Good luck in your search and I advise that you take the advice the others have given and try and learn as much as you can about a GPA. It will help you when you do find one to purchase. I know as that is what I did.
Donovan.