72BBSwinger runs 275/35/18's on his Dart, 18x9 with ~6" backspace. No rubbing.
I've also seen a 275/40/17 on a 17x9 with 6" of backspace run on a Duster, although that one rubbed slightly on the fenders. And most rim designs won't clear the ball joints at the backspace.
The A-body Barracuda has more room up front to the fender because of the fender design than the Dart does, so I don't think a 275 will be a problem as long as the backspace is a little more than 6".
I have a spreadsheet I made up of a lot of the rim/tire combo's that have been posted here and elsewhere on the internet. I know of several folks that have 275's up front with no claimed issues. Most don't run that wide, but I also know of quite a few folks that run narrower tires, but with offsets such that they either sit further out, or further in, than those running the 275's. Which also tells me the 275's will fit with the right backspacing.
For example, the 225/60/15's on my Duster at the moment. They sit on a 15x7" rim with 4.25" backspace. Which means they will have the exact same fender clearance as a 275/35/18 on an 18x9 with 6.2" backspace. Since 72BBswinger also ran his 275/35/18's with as much as 6.5" backspace without frame interference, I know that there should also be PLENTY of room to the frame.
Obviously, not all of these cars left the factory with identical tolerances, so there's always some risks when you're maximizing sizes.