Before you remove the engine or strip the truck down, talk to the local motor vehicles and the highway patrol. There was no set way to record the serial numbers back then, so sometimes the body number was used, sometimes the engine number.
There was a guy on the HAMB that had the car inspected and a new title issued with the body number. He needed a new title in his name, and he explained that he would be replacing the engine (I'd avoid mentioning a newer engine) so he wanted the title to reflect the body number. Research the state laws and see if you can provide them with copies. Show them you aren't trying to pull a fast one, and they should work with you, but do it while all the numbers are still together....