I know it sounds odd, but it was the position of the idler that was the problem with mine. I had a 4# lower and 2.8" upper with a ported Eaton. It started shredding belts when I installed the Metco lower. I moved the lower idler to the middle position, and there were no more shredded belts. I think it was Metco that suggested I move it, but it was so long ago that I can't remember with whom I spoke.
Glad you got it figured out. You've had a lot of patience with the car since the rebuild. Hopefully things are smoother from here on out so you can enjoy the new set up.
I really wanted to do just the idler and just the tensioner alone and see which one actually fixed it but at $40 a pop and already on my 4th belt I decided I like a little mystery lol. Appreciate the tip from everyone on moving the idler.