1988 V6 safari van serpentine belt comes off and gets shredded how to fix

Sounds like one of the pulleys is not aligned with the rest. You can use a straight edge to check the alignment.
I would also want to make sure all of the pulleys are secure and not loose or bent.

Not difficult, but belts rarely just shred for no reason, check all the pulleys that the belt goes on.

one of the pulleys might have a bad bearing, you might be able to purchase a belt kit, check all the accessories turn freely,

Replace the tensioner and all associated hardware. Check the pulleys to make sure they are not warped or have a bad bearing.

If it comes off, check all the pulleys and the tensioner pulley, water pump ect. . why are you replacing it in the first place? if it just "came off" something is bad, belts just dont "come off"

take the belt off and carefully check to see if any of the pulleys are out of line and check to make sure they spin smooth and true check for bearing wear and noise in any of the pulleys cause it sounds like simply there is a pulley out of line

is the a/c working ok??? shredding the belt may be cause by the a/c compressor problems. the "chunk" maybe not, but shredding tells me something does not spin when it should. I dont know why the mehanic put a new pump on that had a problem with the pulley, it should be smooth. anyhting that the belt runs should be smooth and east to turn, make sure that all of the grooves are clean, no remnants of the old belt.

I'd be more inclined to check that idler pulleys and alternator bearings are okay before looking at crank pulley. Take off the belt and give them a good wiggle and spin.

PULLEY ALIGNMENT.WITH BELT ON LOOK AT THE PULLEYS FROM THE SIDE.U MUST HAVE A PULLEY THE IS MISSALIGNED