Ok, so with the brakes and axles bolted in,
Mounting flange of axle to spring 5 7/8"
Mounting flange to inner fender 6 3/4"
Mounting flange to outer wheel well 5"
So you've only got 10 7/8" if you leave the springs in the stock location and don't do any trimming on the quarter lip. That's really not enough to run a 255. The section width on most 255's is close to 10.2". You really need about a 1/2" to the quarters to avoid rubbing, and the springs need a minimum of a 1/4", and that's really tight with a 15" tire. Not as much an issue with a shorter sidewall, but with a 255/60/15 you need to leave room for sidewall deflection. So, you're out of room if all you've got is 10 7/8".
To safely run 255's I think you'd need to run the offset spring kit. Keep in mind that Mopar Performance calls this a 3/4" offset kit, but it's NOT. The front hanger only moves in a 1/2". The rear shackles are offset 3/4", but you don't get a full 3/4" on the front of the spring. It's not physically possible, there's only a 1/2" of room for an offset between the stock hanger and the frame. That's why Dr. Diff calls his offset kit a 1/2" offset. It's better made than the Mopar Performance offset too. I have both, and wouldn't use the Mopar kit again.