if we are talking stock N/A wheels here...i dont think your getting 255's on. my 225's on my Z32 overlap like crazy but i dont really know how those tire configuations work.
i have 225's on my stock N/A wheels and i had 275's on my 9.5" wheels...if i remember correctly the N/A's are 7.5" wide. so 255's should correspond with about a 8.5" wheel but like delerious said, its not that easy.