Max tire size - 68 dart

Gotta measure your own car, that is for sure. While most of them are close if your goal is to max out your tire width then the factory body tolerances are more than enough to sink you.

As for 275's, they need a 1/2" offset hanger/shackle set up. They'd fit on my '71 with the 1/2" offset. As it sits now, there's 6" from the wheel mount to the stock springs and 5" from the wheel mount to the quarter lip. Stock spring locations, stock uncut quarter lips, 8 3/4" with BBP axles. So, 11" to play with. 1/2" clearance on each side means a 10" section width would work if the backspace is dead on, most 245's are ~9.6" at the section width, but 255's are like 10.2". So 255's won't fit most cars without rolling or trimming the quarter lip, tall rear ride height etc. It has been done.

There's 12" from the inner tub to the quarter lip on my car though. Again, that's a '71 and some of the earlier cars are tighter. That's straight across as the top of the wheel opening, so that's not the whole story because the stock tubs taper at the top, but the 1/2" offset would buy enough space for 275's. Again, the backspace would have to be dead on.