Kevin hit the nail on the head with junk yard stuff. Sometimes you can get a decent block, but there are a lot of things that can go wrong. Around here, one of the yards that I use will guarentee no cracks, but only to the point of me picking out another block and trying again. That's on 350s, though, and finding a 327 complete and in good shape will be tricky, at best.
To check for cracks, you really need it torn down and checked, by magnafluxing. That means that buying a bone yard engine and finding a bad one is a lot of work, just to find out you need to do more work. There aren't any great shortcuts, just some time to tear them down. It would be a good learning experience, but it is time consuming and can get a little frustrating.
If you are set on a 327, find a good 350 block and get a 327 large journal crank- (Same block) Buy a couple of books, "How to Rebuild SB", "How to Hotrod SB Chev," "Chev SB V-8 Interchange Manual," or several others. Just do a search and look for SB books. I would get the books first and then start looking around.
Good luck with this project. Just remember to have fun and ask a lot of questions as you go. There are a lot of SB Chev folks on this board that can answer a lot of questions.