RE: Dago Red Mustang and a Jett 60 RE
Ive not seen one with an internal pipe, but its probably possible.
Photos here are a SJ-60L side exhaust with pipe.
If you side mounted the engine, with a riser header you might get the pipe down the side of the fuselage.
On my P-51, I run the SJ-46 and turbo-jett muffler. A few others have run this setup on various mustangs with both SJ-50 and SJ-90L engines.