[AZKen]

Sounds like a plan. Just remember that we all have that trouble with some parts. These truck are old. Way less GMC's built than Chevy. Original Part numbers are no good. We have to use different methods on many parts. Like trying Chevy of the same size truck, measuring and searching, donor parts or rebuild as you say. Don't expect auto parts guys to know. Don't expect books will help. The programmers just went back to where they could get parts, or there was a demand, when they created a data base. Maybe someone here will see your post and have a solution. The next problem now-a-days is finding a shop that still does that work due to so many closing because of EPA, Covid and Aluminum Chinese Radiators are inexpensive.
I like to find parts. So if you want help, post the dimensions and where the inlet and outlet are and their sizes.

If they did have a book and looked in the book and found a part number.............it is obsolete with no crossover.