Lots of good ideas here but the problem statement is to have IROC RSR spacing on the rear outlets which severely limits the options.
I could not find the Fabspeed muffler on their website that Bill likes but I have emailed them to check if it's still available. I would like to find out the overall length of the muffler to see how it fits.
Here's my final design if Jay's onboard. It would allow coming back and installing a larger 22" long muffler if it's too loud and if a larger muffler is found or fabricated:
