I scaled a picture off a ultrathrust and jett sport muffler and they both are almost identical in size for the 61- 91 size muffler. They are both very close to the same shape at the rear end, and although I have not actually seen the inside of the jett, I would bet they are almost the same inside. I also noticed that the top end of a mousse can is almost the same shape as the tuned mufflers of these two brands. I just have to make and weld on the tip section I plan to use a 1.75" mousse can for the outer can and machine a top housing that the can will mount to and that a pipe will slide through. I then will make a rectangle tubing with a flange to bolt to the engine and weld this to the front end of the pipe. I will start with 7/8" od 3/4" id tubing and machine the tubing that will be inside the can down to 1/32" wall thickness. The 1/16" wall thickness will be stronger to weld the top housing that will hold the can and the exhaust flange rectangle tubing. I will make a temporary joint on the can to the housing until I get the right length and then permanently attach it. I can then take 1/8" at a time off of both the can, inner tube, and change the stinger outlet diameter (by adding bushings) each step of the way until I get the best rpm numbers for the props I plan to use.
I noticed that the inner pipe ends right where the taper of the outer can starts. I don't know how critcical this is but I might have to put in an extra step of cutting the inner pipe first check rpm with different stinger diameters, cut outer can, check rpm with different stinger diameters ....... and keep going until I find the right combination.
