Are you sure a leaking PV caused the fireball?
The same thing can happen when a pressure pulse comes up the intake manifold and back thru the carb, followed by fire from a late closing intake somewhere. The venturi don't care which way the air is moving thru it, to unload fuel.
I'm not saying the PV isn't blown, just that you gotta prove it. And again your idle vacuum has nothing to do with PV selection. That cam wants a 10.5 or something very close to it.