1969 Dodge Dart Swinger Rear Tires ?

245 tire is about as wide as you can go. Some cars will take a 255. Maybe lower the back some. That would fill up the wheel well. It seems to sit high.

Exactly!

On a stock Dart, ie, with factory spring locations and no rolling/trimming on the quarters a 245 is already about as wide as most cars can go. Some can manage a 255, but that gets into rolling/trimming the quarters on a lot of cars too. It definitely depends on ride height also.

I would lose the air shocks, if they're filled to the point that they're lifting the car you'll damage the shock mounts. Not to mention they're not great shocks for ride quality. And lowering the car a bit will help give the back end a more "filled out" look.

Since you already have 245's you can just check your tire clearance to the springs and the quarters and see if you have any extra space to go to a 255. Typically you want at least a 1/2" from the widest part of the tire to the springs and the quarter lips to keep from rubbing during cornering or when you've got a couple buddies in the back seat. If you've got more than that now then you can probably manage the 255's.