I don't mean any offense by this, by why would you decide on NES controllers, and not SNES?
The original controller was like holding a box in your hands, where the SNES at least has rounded corners, not to mention more buttons for more functionality in future gaming.
Anyway, you should find that if you mount the controllers at the front of whatever you use for a cabinet, that 6' will be plenty of cord. If not, it'd be fairly straight forward to extend the cable on whichever end is easier.