The hardest thing I ever did, as far as these things go, is weld up the plenums divider in my Edelbrock Performer RPM Air Gap Vortec intake manifold. Would I regret it?
I lost 4 HP and gained a full inch of idle vacuum. Your results may vary. I would do it all over again.
You can go to a shop with a Dyno to find what your engine likes at WOT, but for cruising, for enjoying your commute, for part throttle acceleration, you want no spacer, you want a 4-hole gasket, and you want manifold vacuum to your adjustable vacuum advance canister.
If you can't fit your induction under your hood, you can unbolt your subframe, get longer bolts, and add spacers. I have driven this way, it's no problem with 1" spacers. You may have difficulty with the steering shaft.
My daily carb for the last 2 years has been a FST 650 double pumper because it's no restriction on my 408-HP combo, and everything is easily adjustable or changeable, including the air bleeds. This is what I ran the last time I saw 26 MPH, this is what I used to run lower 13s, and this passed the tailpipe sniffer test. If your area has a visual inspection, it'll fail.
Open spacers hurt vacuum, 4-hole spacers just accelerate the charge into the plenums floor. I would never add subframe spacers to fit any carb spacer.
Lastly, the plenums floor ribs (Edelbrock) or waffle (Weiand) didn't impress me when they first appeared, but now I would add them if I had to use a manifold that didn't come with them. Like if I needed another MPG by adding EGR to my L31. The only carbureted EGR intake for these, doesn't have ribs.
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Is this David Vizard?...