Doug : Can just share what i have when Paul Lamar's forum was still active.
This is what exhaust system and muffler i am goiing install on my Vision Ex project.
It is design so that it can be, build and manufactured, locally at any engineering business
This system will develop the most power as it has no restrictions on flow and are a free flow system.
This Coax muffler (outside pipe) is build out of 0.030 thickness 304 SS the inside 0.035 inch wall thickness 321 SS
the picture is included
I don't know if somebody has build and test these design but a prototype was build and test for sound spectrum
The exhaust runners to the muffler must be flanged were it bolts to the rotar housings.
It will cracked if flanges are weld to the runners
There is also a other design setup but dont know if you are normal aspirated or turbo
Can find this design somewhere if you need it
Regards Le Roux Breytenbach
South Africa
https://drive.google.com/file/d/1TnvV8Ete6d6-MuxY2DPcKDbGjdfF-xwV/view?usp=sharing