The muffler I found from a 1200 wasn't a bolt on part for my 1100, so I never installed it before I sold it. I gave the newer muffler to the new owner and told him that I would assist in making an adapter. I asked him to draw one up and this is what he came up with.
I thought about the stress on that tubing, and the welds taking the brunt of the weight and abuse of the muffler, and decided to do it differently. He is in Alaska on a cruise for his 30th aniversary and I couldn't ask him any questions, so I just did it. Hopefully it will fit.
I started with a piece of 3.5" x 2" x 30" flat bar, cut a small piece off it and did a clean up on the thickness and took 1/2" off the width. Then started drilling and tapping.
Each of the small holes are threaded 3/8-16 and the bolt pattern on the other side matches the smaller bolt pattern. If he can't thread a bolt into it due to lack of room to access the bolt head, I figure he can use long set screws and leave part of them extended, and use a nut on the outside of the other flange. I didn't have a long set screw of that size handy or I would have posted an example to show. The center hole is 1.5" i.d., so should be enough to allow free breathing. I may need to round off the corners, and maybe take some of the weight off it, by hollowing out between the bolt holes, but for now I want to see if it fits ok before I do any more work on it.
Hopefully his drawing is close enough that my work doesn't need to be redone. However, if it needs to be redone, it is a small price to pay for a friend like him.
David from jax