On the wall I have a dual gang box with two L40-30 sockets for each machine, and another duplex socket with a 240VAC 20A and a 120VAC GFI duplex socket.
I assume you mean L14-30. Can you grab a picture of that? I'd think the L14-30R socket diameter would be bigger than a 5-15, 6-15, 5-20, or 6-20 single socket - that would fit under a single round gang plate cover. I could be wrong. The sockets appear to be wider than a single gang. Then again I could go to the store and play around with pieces and parts and see what fits.