Aaron a 10.5 PV will work just fine for you. It's when you get into the lumpier cams that you have to get closer to the idle vacuum because you have so little vac signal to work with to begin with.
BTW - 17" is just fine - sounds like a pretty healthy engine. Did it burp well for you?
Aaron, a 15 will be too big. I pulled this off the Holley website -
A stock engine, or one that is only mildly built for street use, will have high manifold vacuum at idle speeds. To determine the correct power valve the vehicle should be driven at various steady speeds and vacuum readings taken. The power valve selected should have an opening point about 2" Hg
below the lowest steady speed engine vacuum observed.
I have a 10.5 in my 750cfm for my 408ci. It has an idle vac of 17".
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