Possible issues with fit?
 

Im getting a 2012 GT 5.0

Looking to get 315s for 99-04 on a 18x10 rim in the back
And 255s or 265s on a 17x9 in the front

Any issues there?

Ive read that there are no issues on other forums, but I trust you guys more lol :D

I'm not positive because I don't own one, but the offset on 99-04's makes the wheels stick farther out so you may have issues with rubbing on the fender wells.

295 and 305 fit 10" rim better. Not many 18" tires in those sizes so you will be paying a high premium.

The offset is not 2010+ friendly.

Those fitments sound closer for a 17" rim like what I have currently

Thats what I was afraid of...dang it!

I have run m/t 315 DR on mine and i have eibach 2in lowering springs that was on a 17in rim 10.5 wide rear. I am currently running 11.5 inch wide 18 ford gt wheels with 245/40/18 in front and 295/35/18 in rear no issues to worry about. I am going to try out a set of ccws tomorrow they are 18 and are at least 12 inch wide they came off a supra i will update you if they fit

You might want to dig in this site:
http://www.wheelsanything.com/