I have not looked under a B-body like yours but I would be surprised if you could not fit something larger than 4" round. You will want the largest resonator that you can fit in my opinion. The mufflers/resonators are still a straight thru design, so I would never consider them "quiet" with the size motor you have. I sent you a PM with info on the shop I used (rjscustompiping.com) here in Raleigh. My suggestion would be to talk with Ron and let him put your car on the lift and tell you what he thinks will fit.