Scavenging 'Burban doors wouldn't work, the 'burbs didn't get rear side doors until '67, and then it was only on the right side. They got 4 doors in '73.
Chevy Nut, thet GMC that you have pictured looks lik an aftermarket conversion to me. Not too uncommon to see something like that from a construction or mining company, perhaps a loggong outfit as well. I beleive that International made the first factory built crew cabs, most others were made by specialty builders until the early '70s.
