To meet your requirement of stock height, I'd say 255/45 on all corners. It will be noticebly wider and is only around .2" shorter. You could go 275s in the back, I wouldn't though. 265 would be as wide as I'd go on the 8.5" wheel. Alot of the 275s are over an inch shorter or taller than stock, but they ae possible.
I have the stock 18" rims, 08' GT, they currently have 235x50's on all fours. I was thinking on leaving the 235's in the front and going to the widest I can put on the stock rim. Anyone have idea on size also that will most properly match the stock front tire.