Biggest tire and 15" wheel possible on a 1966 Dart?

If you are using the same rim and can measure what you have extra on each side in mm's then you can make a good judgement on what you can run., If yo have a 205 and have 25mm on each side for clearance all around, then a 255 is possible in the same aspect ratio on the same rim. Offsets will move the entire profile in or out, remember the bulge. Looks like the place it will hit are the rear inner fenders, the fender lip (you can roll it carefully) the leading edge of the rear wheel well and of course the fenderwell Hookers you are thinking about in the front ;-)