Steve,
The symptoms you describe (ran rough immediately after backfire, fouled plugs) certainly make it sound like at least a part of the problem is the power valve. I don't know how effective the blow-out protection really is but the engine would be dead rich and the plugs would foul quickly if it is ruptured.
I hope that others will add suggestions based on their techniques to my comments because it's been a long time since I dealt with this problem. I would remove the carburetor from the engine in order to work on it. Check the diagram on the Holley website to locate the he power valve in the secondary metering block. Be sure to drain the gasoline from the carburetor. Remove the floar bowl screws. Be careful to note the positioning of the float bowl and metering block gaskets. The cork ones I remember were fairly delicate and it's important to put them back exactly the way they came out. They can be put on upside-down.
When you have the metering plate in your hand you will see the power valve threaded into it. If you're not sure what it looks like, check the pictures of the various valves on the Holley site. It will thread out of the metering plate with little difficulty (I recall that it took a fairly large open-end wrench to fit it). Carefully unscrew the power valve. You won't necessarily be able to visually determine if it is ruptured. The way we used to check them was to try to draw air through them from the flat side. I know that this will sound relatively primitive but we did this by holding the flat side to our lips to see if we could suck air through the valve. If the valve is broken there will be no resistance to air movement in either direction. If the valve is good, the diaphragm will hold air and the plunger will open and close when you suck on it. No doubt there are vacuum testers available that would be able to give you a much more scientific analysis. This was just the way we did it track-side years ago.
If you suspect that the valve is ruptured, check the flats around the sides of the valve for the number that designates the level of vacuum required to activate the valve on your application. As you can see from the Holley site, there are many different ones available. Your carburetor was designed with a particular one based on the needs of that engine/cam combination. Replacement with another of the same specification would be important.
Disclaimer: If there's a better way to do this I hope others will contribute their own techniques. I don't want to lead you in the wrong direction but that's were I would start. You can remove the carburetor and check this in a few moments, much quicker than checking for timing chains or distributor gears.
Good luck.
