440 dual belts with A/C. Obviously not a matched set of belts. How are you guys putting tension on the alt to tighten these belts? The crow bar/pry bar is just not cutting it.

If the pry bar isn't getting them tight enough then get a matched set of belts

If the twin belts are not a matched set--likely one is stretched/worn a little more than the other--you will likely never get there.

Here at work we have HVAC units that run 4 or more belts on sheave-style pulleys.. If one belt gets broken or damaged we always just replace all 4 as a set, because if you don't you'll never get them to tension evenly and then you will get slippage. If you are having to crank that hard to try to get them to tension, you are risking damaging other things (bearings) just to save a couple bucks on belts. Not worth it.

Also, make sure the pulleys are not glazed. Roughing them up with emery cloth can sometimes solve minor slipping problems.

From whom/where??

Do an online search for matching belts.

Gates V80 series are designed to be matched to a tight tolerance.

CHRGR69 - Also check the pulleys the make sure the sides where the belts ride, are straight and not dished or bowed.
Ron