My Belt wont stay where it belongs!?!?

I am having a hell of atime keeping my belt on my 87. It was coming off when I flexed or went into a ditch or what not. So I tightend it the best I could but now today it came off just going down the road. The PO had taken out the a/c and put in a pulley in its place. Is there anyway to get a smaller belt to bypass this pulley or anyother ideas?
And yes I did a search but its like looking for a Jeep that doesnt need any work. Gonna take awhile!!
Thanks..

What is the condition of the belt?

How are all of the accessories and idler pulleys--any getting stiff/hard to spin?

Check the harmonic balancer.

Sounds like the A/C bypass pulley isn't lined up with the rest of the pulleys. Can you look from the side and see if the bypass pulley is at an angle or too far in front of the others?

Does the belt come off to the front or back of the pulleys? If it comes off to the front it could be the bypass pulley is too far to the rear.

But tilted to the plane of the belt sounds likely as well. Got a carpenters square or level you can lay against that pulley and see if the other end of the straightedge is lined up with the water pump pulley (or any other pulley?)

A missing bolt on the rear of the alternator can cause it to get out of line. The same goes for the power steering pump.

Look closely at their pullies and make sure they are exactly perpendicular to the belt.

if you are REALLY into the idea of a shorter belt, and slightly differant routing...

you could move your alternator where that a/c compressor used to be, on a renix its pretty easy, but you gotta make a short little bracket thing...
pm me if you want a link.

belt is half the length as stock, and half the price.