There was some previous talk in this thread about the PROMAX metering block not having the vacuum port.. and that is what I'm using... about 8 years ago, they (Promax) told me that I didn't need the port for the vacuum advance. They knew my driving -- some street/highway and road course HPDE/HSAX autocross racing... so, yes, it has lots of part-throttle, only occasional flat-out WOT. The car runs great -- and also now with a new SB stroker if still runs great (SB six-pack recently professionally rebuilt).
When I was ordering the Promax setup (base plates, etc, etc), they wanted to know all my specs, etc. They said they would make the metering block to better match my overall setup.... I'm an easy sell.
But... I'm curious if I should put the original OE/Holley metering block WITH the vacuum port back on and see how it would run.
(Maybe the Quick-Fuel metering block WITH the vacuum port would have been the better choice?)
my guess is that promax wants all the info to set-up idle jets and emulsion bleeds. the problem with the factory metering block is the idle jet inside the main metering well. it's too small in relation to the idle air bleed. the idle jet can be replaced or opened up but an easier solution you may want to try for a vacuum advance may be the quickfuel qft-34-8qft metering block. i'm pretty certain they have the ported vacuum provision and replaceable idle jet. if i ever get back into 6paks this is something i'll try. i never did the promax stuff because i never seen the need worth the expense. i just have a do it yourself mindset minus the gadgetry.