I've never told ANYONE that they MUST put 18's on their car. I've told people it's the only way to get 275's on the front, and I've told people it's the best option for maximizing your tire compound selection, but I haven't told anyone, ever, they can't run 14's, or 15's, or whatever. If you want a car that can handle corners, you're at a serious disadvantage if you stick with anything smaller than 17's, but that takes more than tires anyway. Slapping 18's on a car with stock suspension would be almost as bad as slapping 15's with BFG T/A hockey pucks on a car with suspension set up for handling.
The OP's car looks very stock, and he didn't ask about going bigger than 15's or improving his suspsension, so, why would I say anything about 18's? Staying SBP pretty much eliminates that as an option anyway.
Assuming that car is stock, a 10" wide rim probably won't even fit into the wheel well without a tire. Stock Darts only have about 11" from the springs to the quarter lips, and that's how wide a 10" wide rim is, outside to outside. The 10" width is where the tire seats, not the outside lip.
The pre '70 cars tend to have a little less room than the '70+ cars. It's not a hard rule, but the measurements I've seen from the pre-70 cars tend to run on the narrow side of the body tolerance spectrum, while the '70 up cars tend to run toward having a little more room. Obviously there are exceptions, but I wouldn't recommend 255's for a '69 Dart unless the owner was willing to do some work on the quarter lips or run a 1/2" spring offset.
