I highly recommend Burns Stainless
Burns Stainless: Performance Exhaust, Collectors, Race Mufflers
BURNSSTAINLESS.COM
High Performance Stainless Steel Exhaust Parts. Burns Stainless offers exhaust parts, tubing...
I ran their muffler on my race car and passed all sound limits including San Diego where they were a bit rediculous.
You get to pick the internal diameter, external diameter, andlength. They are also serviceable and can be repacked. They are super light: 3 to 4 pounds. They are not cheap, but in this situation you get what you paid for.
Our rules were that the exhaust has to exit behind the driver seat, so I ran the header to a short pipe to muffler, another short pipe and turn down right after the driver seat. I did not take it to the side.
Steve K.