73 dart 4 link install ???

As already stated, the frame rail isn't going to determine whether or not you can keep the seat, it's the wheel tubs you use that will decide that. With a back half you use aftermarket tubs which will be wider but also longer, the longer is what is going to eliminate your rear seat. When you mini tub your only widening the tub not lengthing it. More power to ya if ya figure it out but I've never seen it done with aftermarket tubs.