I had salvage liners installed 60 to 70 years ago and it was a cheap but lasting fix. It is still a reliable fix, but I would expect it to be expensive. And before you go machining the deck and installing a salvage liner, it would be wise to have the cylinders sonic tested. After 80 years the block may have rusted from the inside. There is a guy near here https://www.eatonbalancing.com/ that does that. He is big into Y-Blocks, but he would test a GMC also. I would look around for a better block. If you are just looking to build a spare you have plenty of time.