Maybe try another locksmith? Is it extremely corroded to the point it wouldn't work with a key? There are only so many key combinations that exist for the 1936-up GM locks. Dealers used to have complete sets of keys. Some locksmiths do now.
Isn't there a locksmith right here on the forum?
My favorite locksmith fixed my button a long time ago. It had been hit and bent into an oval, yet still he managed to fix it. That was probably 1987 and it still works. I would send you to him, but unfortunately he has passed on.
As a longtime clamshell owner IMHO put the barn doors back on. The clamshells are cheap and flimsy, and if you were planning to use it as an actual truck tailgate (like I do), you are going to be disappointed. It is nice having a tailgate, I'll give ya that. Mine was all broken up and repaired badly when I got the truck, and it is worse now. I can't even figure out how to begin fixing it, because there is no solid metal anywhere, just thin sheetmetal like a door.
Good luck getting your lock fixed!
https://www.youtube.com/watch?v=pRva7z8pvwc