Spitting off the belts?

I ran a 340/418 stroker at the drags yesterday.
First run it spat the power steering belt, and turned the water pump belt inside out.
We left the power steer belt off for the day. (After all, that power steer pump is stealing horsepower)
But next run it spat the water pump belt right off. (Fortunately it stayed on the engine, I left a supercharger belt on the start line once lol)
So we raced off to a parts shop to get a new belt.
Third run the belt stayed on fine.
fourth run it spat it again.
All the pulleys are factory ones. The alignment looks pretty good. It anything, there could be that one real thin shim missing from behind the water pump pulley.

Could that be enough to make it throw belts?
It's not a big ask to make new crank and alt pulleys from scratch for it. (Have a fitter and turner at the ready)