You stopped for a stoplight.
At least I think that'* when it happened, please confirm.
When you stopped and with all the electrical load, AC etc etc running that the engine rpm dipped below the alternator cut off that is set in the PCM. This causes the alternator to literally turn off. Wtih all the stuff you had running, it'* no surprise that the voltage gauge dropped.
The tensioner, alternator and tensioner bracket do not need to match the year of the block. I've mixed and matched these pieces with 100% success in the right combinations. Please post a picture of your alternator to bracket. This is likely an easy fix up. However the larger issue of the voltage drop isn't the bracket, alt or tensioner unless this is a slippage issue and it didn't happen at a stoplight.
There is also another mod that may add voltage as long as the alternator is being commanded on by the pcm that we could have you do. But let'* back burner that until we can get you up and running properly.
I'm going to go reread the post as I may have missed a couple of things.
I'm back! Think I read into things a bit and may have some other ideas as well.
Please confirm that you have a 99 LeSabre alternator bracket and tensioner and the 95 Bonneville alternator.
If that is correct, then you have pretty much the same setup you had from the factory. How is your belt routed? Did you route it intuitively and then lift the belt off the water pump to the tensioner to be tensioned? (That is the correct way). Being that it'* shredding belts, something is out of place, whack or some odd thing. Did it just start shredding after the UIM change? If so.. let'* review how the pieces sit, if the belt is straight etc.