[scicala]

Yes, you can just use only the inner diaphragm on the dual diaphragm distributor, but any later carb will still supply way too much vacuum to the diaphragm on the '56 distributor.
The outer diaphragm is just used to retard the spark slightly under acceleration momentarily to avoid spark knock.
I also currently have three freshly restored Holley teapots that are correct for '56 if you are interested.

Sal