I suggest using Lusid's plans. It offers one of the larger control panels available and is pretty simple to do. His templates come with the SFII button config and can be placed wherever you want on the panel.
http://users.adelphia.net/~seanhat/arcade/
I personally don't have any intention of making something that large. I've considered a 3P, but any of the games that require more than 2 players only need 3 buttons per person. X-Men, TMNT, Simpsons, Avengers, etc. None of them need the 6 button config. Good luck and let me know if you have any q's.