The spacer that Speedway sells is made for a Holley 390 CFM 4160 type carb - a "modern design" vacuum secondary version. The original four barrel carb was a Holley 400 cfm - with the (4) 1-7/16 throttle bores that just match the "B" manifold. The base plate on the two carbs (old and new) is an exact match. Some 50's models of Mercury used this same "B" manifold with an AFB type carb like the Edelbrock.
This modified "B" was set up by my friend Charlie McCraney. It is opened up as a slot side for side - but the center wall is maintained to keep the 180° split (every other cylinder feeds left or right).
I put a Road Demon Jr. 525 on mine - a vacuum secondary 600 cfm is about the limit for a responsive street driver carb on a Y.