My '71 383 factory A/C car would throw the belts at 6,000 RPM. I tried several brands of belts and align the pulleys and nothing worked. I did read that the original belts were 11/32" wide? and all the replacements are 3/8" wide, so they ride up higher in the pulleys. I bought a March underdrive pulley set and have not thrown any belts since, but the underdrive ratio slows the alternator down so much it does not charge unless the engine is at 1,200+ RPM.