1969 dart wheel and tire size?

Have you converted to large bolt pattern or are you still 5x4. This will dictate your offset/backspacing for wheels.

Also, what fits a Barracuda, Duster, etc in a lot of cases will not fit a 67-69 Dart. Even 70-up darts have more rear room. It is best to measure because as you've seen, it can vary. I have 245/45/17's on the rear. with a 17x8 wheel with at 5.25 back space and it's tight.