Charles,
I have used the smaller ID carb from the 67 CMB motors on 67 Macs. If I get time, I will see if an extra 67 CMB carb I have on hand will fit the 67 Mac motor.
In the past I got my SG 67/80 Twin to stay running with the CMB carbs on the 67 Mac motors. At the high altitude in Colorado the SG Twin with the standard Mac carbs would go about half a lap then one or both motors would quit.
I had the same type of problem with an A rigger Roadrunner & 3.5 CMB with it running for a while and then quitting shortly after running around the lake. This continued until I went to a smaller ID carb!