Just depends on how crazy you want to get with the front tires or how much you want to push the fenders out. But on your car I don't think you'd have a problem based on the combo you have on there now and the fact that you've already worked the fenders.
Do you still have 245/50/15's with a 15x9 and 4 1/8" backspace? If that's what you have now you could totally run 18x9.5's with a 5.25" backspace and 275/35/18's, even with the 13" DoctorDiff brakes. Your distance to the fender would actually almost the same as your current set up. The only wildcard is that a 275/35/18 is taller by about an inch, so, your clearance to the corners of your fender openings would change and on a Barracuda that can be an issue. But I know you've modified your fenders already, so it might not come up.
