How early B Body?
I've used Summits 3.0 inch kit (no X or H pipe) on my 1966 Dodge Coronet Deluxe 2 door sedan with the springs move in and it fits very well, I bought a set of Walker 3.0 muffs and they are fairly quiet in the shop, I haven't drove it out on the road yet as I want to take it for front and rear alignment before driving it after a complete frame off rebuild from the front bumper to the rear bumper. That kit does not come with muffs.
Some of their kits do come with Flowmaster type muffs
, I have their 3.0 kit with the Flowmaster type muffs on 1969 Darts GTS 383 4 speed car with the leaf springs move in that I ended up mounting the muffs behind the rear end between the rear sub frame and the outer 1/4 panels, fits well and looks good. Mounting the muffs back as far as possible helps reduce the exhaust noise in the car also
I don't like the sound of the Flowmaster muffs
I used stock gas tanks in both cars and I had to dent the passenger side of the Darts tank, to clear the leaf springs
I had to massage the lips on the 1966 Dodge tank to clear the leaf springs and rear spring hangars