[k1787]

I apologize if I am not answering your question, but.........Single diaphram tach drive distributors were available on 55 and 57 tbirds. The 55 diaphram is approx. 4.5 inches in diameter, while the 57 diaphram is a lot smaller. You will have to change the vacuum lines that ran into your 56 dual diaphram when you replace with a single diaphram? Not sure what you will accomplish with your distributor swap. Why can't you put a single advance on your 56 distributor instead of buying another tach drive dist. b