Nothing more is needed than using a stock factory 1/4" thick gasket.
I've done a LOT of intake, and intake/spacer testing, enough to write a book on it.
RAISING the carb up on a dual plane intake can cause issues even if it makes more power clear up near the shift point. You have to be careful as to how much area you allow one side seeing the other side. Those intakes were divided for a reason, even aftermarket dual plane units.
I've added open spacers to them with no other changes and immediately ran into "stumble/hesitation/bog" issues going quickly to full throttle off or just off idle that would NOT tune out.
I've also had to add dividers back to intakes where the well meaning tuner ground them down or completely removed them for the same reasons.
IF you play around with spacers on a dual plane intake forget a four-hole or fully open. A fully divided or semi-open may help some. I found some top end power using them but even with a stronger top end charge I lost enough 60' and short times my car ran quicker with no spacer at all. Think about that when you throw in all the "cobbling" often associated with raising the carb way up in the engine compartment.
I'll also add here that you can loose a LOT of power with drop base air cleaners if the move the lid too close to the carb. Did some testing there as well. So even if you pick up some power with more plenum area you have to make sure not to choke-off some power restricting air flow into the carburetor........FWIW......Cliff
