Free PCB software (for small projects):
http://www.cadsoft.de/I did just one like that for several years ago for my A500. As I remember, it was just a question of enabling one of the two ROM's. All pins was basically parallelled except for one of the chip-enable signals that was routed to a alternating switch. Making them low enables the chip, making it high turns the chip off (logically). I don't have the schematics available as this was before-internet time... (it was on some paper that has been lost)
