If I could, I'd put a electrical shock device in your mouse right now to deter you from even thinking about doing this on just about any application. Think of the power valve as a switch that richens your mixture for heavy loads and accleration. If you decide to plug that power valve, you will need to increase the size of your main jets for proper full power mixture to the point that your cruise mixture will be so rich that your engine will actually foul spark plugs. Too much gas is just as bad as not enough gas. The engine will be sluggish at low rpms, and just won't run right. If you have problems with power valves blowing out, fix the problem, or install one of the available power valve protectors. Summit sells one for about $8. All 4010 and 4011 Holleys, and many newer 4150 and 4160 series carbs come from the factory with this power valve blowout protector.
I'm running a power valve plug right now. My motor makes 8 inches vacuum idling in gear and it will open a 3.5 power valve just off idle which is obviously way too soon. I got tired of it blacking up my plugs.
Low idle vacuum caused by a big overlap cam is the only situation that warrants a power valve plug.
I have about 11"'s of vacuum and figure a 5.5 or 6.5 would would do the job. I've read two different methods to figure out which power valve to use: (1) divide your vacuum reading in half and add .5 and (2) Use a power valve that is about 2" below the vacuum reading you get at idle and in gear.
With 11" vaccum I figure 5.5 or 6.5 with the first method and
with the second method I should be using an 8.5 or so?