I've been all over that country. I grew up just over the border a little way in Idaho. Malheur Country used to have an emergency call box system that I helped install back in the 70's. Back then I must have been on every road east of 97 that shows on a map and a lot of roads that don't.
Unless its been buried in a river bed I'd expect it to have been pretty dry, even when covered by snow.
It doesn't sound like you're making lots of changes, like hot rodding, and it doesn't sound like you're trying to restore this truck to original condition. It sounds to me like you want to make it complete and looking generally like a truck of its time.
If I've got that right I'd say you have a lot of interchangeability available. Many of the things that are different really don't matter except to an expert and they will not affect function of the truck.
I say grab that truck! Of course I already own two 1-ton GMC trucks and might be a little biased...
