My 68 Dart is off center compared to the rear body.

BUT REALIZE, you are just centering the rear end to the body. The body is just along for the ride. The position of all four tires relative to each other is what matters for the car to go down the road straight.

That being said it's probably not going to be down to 1/16" or something. Alignments alone change wheelbase and track width some. The service manual has some guidelines, but you'll have to look at your last alignment sheet.