Unfortunately, I don't know if there's a definite fix for your situation. Speed runs are way outside the performance envelope the car was designed for. Although the Matt Francis spur adaptor MIGHT help....it's also possible it would simply make the problem worse.
It seems to me the biggest culprit is probably the belt lash. In other words....at those rpm's, the belt is probably trying to fly off the pulleys so belt tightness would be critical. I believe you'd have to have it tighter than normal. You might also consider making a mod and adapting another belt tensioner ( or 2 )forward of the existing one. Possibly one on the top of the belt and one for the bottom.
Just a thought.