Welcome to the forum
I agree, looks like a good solid truck to work with. That's unbelieveable that it has been sitting in a garage for the last 20+ years. I see by the plates that it was originally from Twin Falls, Idaho.
You may get lucky and have 4.11 gears in that Eaton HO52, but more than likely it will have the 4.57 gears. That is what my '66 C20 has as well as the No-Spin differential. Have you checked to see if the SPID is still on the inside of the glove box door. That will show you what the truck originally came with.