Ron,
i once had a 56 that i bought in Calif, and it was smoged.
You cut a hole in the air cleaner to get the source of vacuum at the pcv valve. I still have an air cleaner, or possibly two, that have this mod. Not sure if the hardware is stii there.
There is a new pcv valve being offered for 1956 cadillac on e-bay, i don't remember which search it came up on, but try pcv vave or such.
john w