255 is pretty tame. I have 255 up front right now on 8 and that looks fine and is nowhere near the size I would go with on custom wheels. 18x9 or 9.5 will work and can easily support 275 with a minimum amount of hassle. +20 does sound like you are getting quite aggressive. might want to dial that back a bit. I know +35 fits, but im not sure how much space you've got on the fender at 35. I'm not certain of the offset at 9.5. Glen has CCW customs at 9.5 front. no offset listed though. Maybe ask him. [ http://www.twinturbo.net/net/directory.aspx?forum=general&user_id=5206 ]
Out of curiosity, what wheels are you looking at getting?
Antihero
Putting AMS garbage, and $500 ebay wheels on a Z is like dressing a Victorias Secret model in a Walmart dress and Payless shoes. You're embarrassing yourself.