so far as a serial number, I don't know if I would call it that. I don't think the engines were coded with a specific id#, but the casting number should be visible. You just might need to scrape through some gunk to expose it. On my Chevrolet inline, it is located on the passenger side of the block. You can also pull off the valve cover and see the head casting number stamped underneath the rocker assembly. The head doesn't always match the engine but the numbers could prove useful down the road. I am not sure if GMC stamped their casting numbers in a different area, someone else will chime in if this is the case.
So far as the vacuum advance is concerned, most if not all the inlines that I have seen, have always had a mechanism. They are not hard to come by, many of the fine links listed on this site sell them. Again, someone else more knowledgable will probably chime in on this. Good luck.
