Rear Duster Tire Size

Just because you can install a 245 on a 7, doesn't mean that you should. With optimized air pressure, it will lay the same skinny blackies as a tire one or two sizes smaller. And if that optimized pressure is too low, it will handle poorly in the corners.
IMO, with a BB under the hood, you can't afford to fool around with skinny tires. You are just inviting trouble.
Wheel Vintiques will sell you a couple of custom backspace Rally wheels pretty darn cheap.

245s are not adequate.
I have a 367, in a 108wb, 68 Barracuda, which is pretty much identical to your Duster in every way that matters. My first set of tires were 245s.
I lost track of the number of times I spun out with those 245s back there, going around a corner and gassing it just a tiny bit too much.
I leave it to your imagination what your 440 will do.
Spun-out means broke traction and lickitty-split spun around 180*; usually putting me up on the median, with cars passing, and another broken alloy wheel. When it starts you have just micro-seconds to correct; after that you are just along for the ride, so hang on.