if you are certain that the tensioner pulley and alignment are cool...as per
your previous post (its always a bummer after about two belts, and the guy
scratches his head...lol) The next place to look is special.
The front of the block has a yoke shaped plate mounted to it, upon which
all the ancillary devices...alt.,p/s pump, smog pump, etc. are mounted.
If you can imagine looking at the setup from the front, the center pins (capscrews) are placed into the block, the outer pins sink into the heads.
I would wager one of those outer pins (capscrews/bolts) is broken.
most of the time the idler pullet is the culprit, but it is another part of this
assembly if it fails rapidly in succession.
If your serviceman is unaware of this common problem with every GM block
made since the 80's I would give some serious thought to finding one with
some experience.