If I ever have the time and space to build a full size machine it will probably be a Multi-Williams. I really dig the artwork (both designs) and all the Williams games rock.
However, I'm not sure you'd be happy with a bartop. Defender, Stargate and Robotron are all very physical games. To really recreate the arcade experience you have to be pushing against something sturdy when you move the joysticks.
I haven't tried the 360s but if their feel is anything like the standard Sanwas then I'd say they're probably too soft for the Williams games. Have you considered using Suzo Inductives?