[Jack21]

Without researching muffler manufacturers, J.C. Whitney sells the 36" x 4" x 2 1/4" glasspack blanks. Cut out the existing ones, and weld in the new ones. You'll have to shorten the collector, and turnout. Install a 3" - 2 1/4" adapter on the muffler inlet for transition flow.

Another tip would be to weld in a collecter flange after the J pipe so you can replace mufflers easier. You can then ceramic coat the inside and outside of the J pipe, just like the headers.