The 8.1 liter gmc gets 2 for 1 mufflers.

Replaced the expensive gm dual inlet and outlet muffler with 2 flowmaster style replacements.  They fit and sound very nice.  Way more mellow than anticipated.

Mufflers.
https://www.summitracing.com/parts/su...

Tubing expander.
search for: Astro Pneumatic Tool 78835 Exhaust Pipe Stretcher Kit
https://www.amazon.com/Astro-Pneumati...