Jerry Hey wrote:

 Is there a significant performance gain with the tuned version?  I
 have got the impression that exhaust pipe length is not so important
 as unrestricted breathing.    Has anyone built one of these mufflers?
 Is the simple version just an empty can that the exhaust dumps into
 from the side?   What type of Stainless?  Is ".040 inconal" a
 complete specification?   Jerry

In theory yes Jerry on the tuned version. Not to my knowledge has any
one built one yet. I am not sure it would fit in your cowl as the diameter
is about 10 inches while the engine is about 6 inches from center line
so the width of the cowl in that area would have to be about 32 or more inches
wide. Brian Trubee designed a simplified version with square runners welded
up from flat sheet that you might want to try. It is possible to make it
slightly more compact as the runners could be rectangular.

Just an empty can. .049 thick 316 stainless ought to be about right.
0.065 thick 316 SS would probably make it bullet proof.
Inconel (pure nickel and chrome) is great but expensive.

Paul Lamar

Jerry Hey wrote:

Paul, it is easy to find 6" O.D. SS tubing remnants in this area.
It is used in the vinyards. It is type 304.  The gentleman I talked
to thought both 049 and .063, were available,  but he did not want
to walk out the the remnant pile to find out.   I don't know how 304
stands up to the heat.   Very hard on tools as I recall.   Jerry

304 will work. Not quite as good as 316. There are tricks to 
drilling SS. As I recall you need to keep the pressure on and
keep it cutting till it is done. A plasma cutter is no problem
however. Tracy just bought a new one. You might find a sheet metal shop
that does commercial kitchens.

Paul Lamar
 
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