Wow, what perfect timing! I'm in the same boat actually. I just got some Mac Performance 2-into-2 taper tip slip-on mufflers to replace the stock mufflers that had completely rotted out on my 81 XS400S.
These are them:
http://www.motorcycle-superstore.co...acement-Mufflers-1981-Yamaha-XS400-Parts.aspx
I removed both header pipes and the part where the mufflers mount to the frame today after I got home from work.
Man, getting the flanges off from the engine was tough as hell! Aside from that, very simple and straightforward. I only needed 10-12mm wrenches, a crescent wrench and some serious elbow grease to get those flanges off.
I also noticed that the stock mufflers were welded to the header pipe. I thought I was going to get off easy, and just pry out the mufflers from the header pipes. Nope!
My kit came with some clamps to fasten the mufflers to the header pipes. I think what I'm going to have to end up doing is, very carefully cut the muffler from the header pipe, then sand and polish the header pipe before fitting it into the muffler and clamping it. They key here is going to be making sure I cut it off at precisely the right point, so that it fits snugly and still is able to be bolted on where the muffler meets the frame. I'm planning on giving it a shot tomorrow after work, so will post my results as well as pictures and commentary.
Kindest, for my mufflers it looks like I would have to cut them off about an inch or two to the right of the hole in your picture.
Depending on how it looks and sounds, I may have them welded on later.
Another concern is whether I'll have to rejet the carbs after to make it run well. Will have to wait and see how they work after I get them on though.
If anyone has any other suggestions, please let me know.