The holley power valves are labeled by their vac rating. For example, a 65 opens up when the vac drops to 6.5". Normally, you want to run a power valve that's rated at about half of your idle vacuum. Hope that helps.
As for the vacuum going up with RPM....
I dunno. The idea about the carb being too small makes sense, though; like trying to suck a milkshake through a straw, you build up a high vacuum inside the straw unless it is big enough. I run a 351C with a moderate profile cam, and a 750 seems perfect, so if your cam is huge, it may do better to with a bigger carb.
The thing that confuses me is that the power valves open at low vacuum and put extra fuel into the mixture. So unless your jets are super rich, I would expect you to be running lean since your vacuum is so high. That's obviously not the case, though, since you keep fouling plugs.