Riddle me this...

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 leaking valve 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.
If you wanna prove it, put a PV plug in there, and plumb a vacuum gauge to the manifold and run it up to the windshield and clamp it there with a wiper blade.Then take it on a roadtest. Get it into second gear, above your stall speed, and roll on the throttle till the engine lays down, then grab a vacuum reading. Do a couple of those and average them out. Then move up 500rpm or so and repeat, and one more. then go home. Now average out the averages,lol, and add .5 to the nearest .5. Say you got 10.0; add .5 and a 10.5 is what you want. This is your starter PV.
If your engine wants a 10.5, and you install a 6.5, then she is gonna be lean all the way from 10.5 until the PV opens at 6.5,not good. You are gonna feel that sag and do one of two things; 1) press harder on the gas pedal to get past it, or 2) install fatter mainjets.. Now the bigger mainjets will get you thru the sag, but two other things are gonna happen; 1) now when the PV opens, she will go rich and not make the power she should,and 2) she is gonna suck gas all the time cuz your MJs are too fat.
Does any of this sound familiar?
Ok but you realize the fatness so you decrease the secondary MJs, and it seems to work pretty good...... until the plug-check shows the rearmost cylinders are now lean.
Ok just put a 10.5 in it and prove me wrong,lol. Maybe you can take a jetsize out of the primaries. Maybe She will smooth right out and be happy,lol.