I would go with the three inch system to start with, I also wouldn't have bought any brand of Tri Y headers , but that is me I had set of 2.5 inch pipes coming out of a set of Hooker A body 1 7/8 inch A body fenderwell headers into a set of 2.5 Turbo muffs and then a short set of tail pipes(side pipes almost) on them exiting in front of the rear tires on my 1969 Dart GTS with a mild 440 bracket motor, that car ran 3 runs in the 12.2s E.T. at 109.+ MPH with exhaust hooked up at the old LACR drag strip in SO CA on Friday night, I removed them and the car ran 11.39 at 117.+ MPH on the next three run I built another street and strip car several years later and put a three inch sytem on it with the same headers and that car would run 7.02 in the 1/8 mile at 97.3 MPH through the exhaust sytem that ended at the rear bumper on that car, 1971 Duster with a pump gas 518 C.I. low deck stroker motor, that car ran 6.98 at 97.5 MPH with the exhaust system removed I ended up figuring it was the weight of the exhaust system that made the car a tiny bit faster, not the exhaust sytem restrictions I had the exhaust shop used aluminized 3 inch pipes and a set of 14x7x3 side inlet and center outlet oval Magnaflow muffs mounted behind the rear end exiting at the rear bumper That system flat worked well BTW I used the stock Duster gas tank on that car, but the head pipes from the headers had to go under the rear end (do to the coil over shocks blocking the stock pipe locations over the rear end housing ) and then bent in and up to go between the stock inner sub frame and the outer 1/4 panels I ended up buying the old dart back and bought a 3 inch exhaust system from Summit and had it welded into place the same way, it has the leaf springs moved into the sub frames so the muffs and tail pipes are ran the same way as the Duster but the pipes do fit in the stock location and those pipes go over the rear end housing into the muffs beside the 1/4 panels Those Summit brand muffs sound a lot like and look similar to the Flow Stoppers muffs