I am in the process of building linkage for my GMC with Hydramatic for the 2 carburetor manifold. It takes a bit of engineering, since I have Fenton headers and the manifold has the carb. flanges turned 90 degrees from the way the factory is. The linkage interferes with both the headers and the manifold. I am in the process of engineering a linkage that will clear everything also will be progressive, the 2nd carb will start to open when the primary is at about 1/2 throttle. Oh the fun of making hot rod stuff, reminds me of how we did it back in the '50's when there weren't hardly any aftermarket parts made.