I had an old locksmith key all my locks last year. He claimed to have all the GM keys, by number, between 400-500 different numbers. He says that if he had the number, he would get the correct key. This was for my '53 Chevy, I assume that GMC would be included. I just re-read your post, he needs the number that is in the stamped out part of the key, I don't think the lock number would help.
