[Willys_MB]

Is your alternator tight? If it's not tight enough it could loosen up enough to let the belt loose. You could also go to NAPA/O'Reilly and get a belt that's 1/2" shorter. You're currently running a 53" belt, so maybe try a 52.5" belt. Without pictures it's really hard to help figure this out. If you can post some pics of your setup we might be able to help you out.

Belts suck. They're just the sort of thing you have to trial and error. You can get a ballpark figure using a string, but you still need to get a belt and put it on there to really know.