Well....it depends on wht your intake is and what its designed for. All wrenches are diff. on this thought. I use the motors VE and see what its airflow capabilities are, then go from there.
Now, that being said, we've tried to talk it out over at SpeedTalk, we've all got diff. ideas where it helps....sometimes it surprises us! Heres a little food for thought....and read the whole thing, you might be pleasantly surprised....
http://www.automedia.com/Carburetor_Spacers/pht20080701cs/1
Now then, I've calculated the volume of the main chambers and runners and have actually welded lots of spacers onto the intakes and have gone over to the flowbench to get more power. Final product was like the Accel pro-ram....more or less a over glorified version of a Scorpion intake.
It can be done also with dual planes, such as what your talking about.
If you need the spacers to get your carbs on the intake you need them. The extra performance they'll give you is a added bonus. You might have to change them around for optimum performance. But....you can get a slight increase, as I said.
Never mind me though....I've just been a top wrench on several Top teams before opening my own shop....and all the years I've been doing it, theres one piece of advice I can give you...... just to try it!
[P