Well, I know when I put on the belt that was 1/2 inch shorter it was a tight fit. There wasn't a lot of play left in the tensioner after it was installed. The stock belt fit nicely, but once the car started I had similar problems. After just a few seconds it started to scream somethin fierce!
Ok, I just went outside to get the actual part # from the one that is on mine.....and guess what. It isn't labeled.
I belive the first belt I used on it was 1/2" shorter, and it worked great. When I needed to replace it I found that the same belt (part # anyways) was too short, and a OE belt fit.
I can't explain it, but that'* how it went.