1969 Dodge dart rear tire fitment ???

It'll be close, and it will depend entirely on the backspacing. A 255 is usually a stretch for a Dart with the stock wheel wells and spring locations, especially the pre-'70 cars. With the quarter lips rolled flat it will probably work for you, but your backspacing is going to need to be spot on. And if you're running an A-body 8 3/4 with BBP axles that means you'll probably need more backspace than the stock wheels had, most of the stock 15x7's had a 4.25" backspace, which is a +6 offset. With A-body 8 3/4s and BBP axles you typically want more like +12, which is 4.5" of backspace on a 15x7. It's 5" on a 15x8, but that's pushing the limits for backspace with the stock spring locations even with the BBP 8 3/4. But it's better to have a little more backspace and correct with a small spacer than not have enough and be into the quarters.

When you get the springs on you can use a square and a tape measure to figure on the straight line distance from the springs to the quarters. Once you've got that you can subtract about an inch (1/2" clearance on each side) and that will tell you the max section width you can run. On my '71 Dart I have 11" from the stock spring locations to the quarter lip, but the quarter lip is stock. Rolling it flat would give an extra 1/2" or more on my car, so, the 255's would work with the right backspace. But that's my car, and the pre-70 cars tend to be a little tighter for space. I also have 6" from the wheel mount to the spring on that car, which has an A-body 8 3/4 with BBP axles and drums. If you figure a 5" backspace plus a 1/2" of tire overhang that gives a 1/2" of clearance to the springs. On a 15x8" the tires you listed would have a .6" overhang (assuming the measured rim width they used to get that section width was a 15x8, which it probably was), so that would be about as tight as you wanted to make it to the springs by the math anyway.