Throwing pulley/alternator belt

I have a 340 with A/C, belt alignment is perfect, you can lay a round steel rod in the groove of each pullet & see where the true aligned point is... This 340 even with perfect alignment would toss the belts every time I spun it past 6300 rpms... I tried different brands of belts, I tried extra tension.. Didn't matter, it tossed the belts...

Then I tried an old hotrodder trick, I added a long bolt to the lower alternator bracket, it doesn't normally touch the back side of the belts but it's less than 1/4" from them... When the rpms go up it touches not allowing the belt to grow away from the alternator... Haven't lost a belt since then & I had a 6800 rev limiter on the engine for a few years... Oh, 4.30 gears & Edelbrock heads so it breathes decently...