Well the price was rite!
It may be worth throwing a new set of belts at it, the ond ones could be worn?
Any Polaris atv or snowmobile dealer could give you a few tips on variable belt drives as all there atv and snowmobiles use them.
It is basically a matter of balancing the centifugal (RPM) activated weights against the force of the springs. One in the drive clutch and one in the driven pulley. I have no particular on your machine but have dealt with atvs and sleds using a belt drive.
Make sure the bushing that the sheeves slide on are not wollowed out and that they have some lubrication (no so much that it gets on the belt) These need to slide freely, without binding so the upshift and down shift are smooth and predictable and controled only by engine speed and not friction.
The springs that hold the clutch disengaged at idle and control the upshift from low to high ratio can get weak over time and allow the upshift to come at a lower engine rpm causeing bogging the engine.
Take the drive and driven apart and clean and lubricate the clutch and driven pulley. Look for worn weights, weak or broken springs loose bushing and any binding that prevents the smooth movement of either pulley.
Also it is good to have a new belt to compare the % of wear of your used belt against. This is the most common problem. As the belts wear down they come “higher“ in the clutch before the drive engages and lower in the driven. (the belt acts too long) Which results in a loss of your low gear and your high gear ratio. (you only have the middle ratios left, sort like starting off in 2nd gear and missing overdrive, which results in more belt slippage and a higher wear rate and poor performance)
Hoffco Comet Industries http://www.hoffcocomet.com/
made many of these belt drives for all types of vehicles and may be a source of parts and information. I've found their tech people quite helpful, though they are geared more to manufacturers then to end user. If they didn't build the drive for your mustang, there is a vary good chance they could supply a replacement should that become necessary.