I actually went the other way with my M1 dual plane. I have a 1/2 inch wood spacer with a divider and it came with 2 open carb base gaskets. I cut 2 new gaskets that have the same center divider as the spacer. And I modded the carb bottom too. I'm using a Edelbrock carb and it has a large open hole under the air cleaner rod that allows air to move back and forth between the primaries and the secondaries. I filled that hole and also the vacuum port at the back by the secondaries. The idea is to stop crossover from one port to another. The result is I have 2 inches more vacuum at idle and a crisper throttle response.