Biggest rear tire in a 68 Dart?

anything bigger than a 255/60/15 on a 7 1/2 wide B-B rim with 4 1/2" back spacing will be pushing the limit (and maybe beyond).
You will have 1/2 - 3/4" clearnace on the inside from tire sidewall to spring, and maybe 1/2" to the wheel well lip. That all sounds like a lot of clearance until you start going around cloverleafs with 4 people in the car - then it is not.
Mine has 215/65/15s on 6/12 " rims on the front and 255/60/15s on 7 1/2" on the back so the stance is close to being level.
Too wide on the front can lead to front fender interference when turning.
Hope this helps.