Just installed 650 exhaust on my 550, but it took a little bit of work. As TX650 said, the pipe diameter is different, but they are perfect for cutting and combining. Here's how I did it, since I found this question asked a few times when I searched for the answer myself.
I cut the 550 head pipes 8 inches down the pipe (measured on the inside of the curve)
I cut the 650 header 6 inches down each pipe (again, inside of the curve)
Slid the 550 head pipes into the 650 header
-keeping each head pipe in the position from which it came [ie: cylinder 1 stayed with cylinder 1, etc] not sure if this is
necessary, but there did seem to be some subtle difference between the various bends
-some of the 550 pipes had to be squeezed and "worked" with a hammer at the cut end to fit inside the 650 pipes. nothing
major, just minor adjustments. Some of the 550 pipes were cut near a bend, which made the pipe not exactly round
I bungee corded the exhaust to the bike (650 bracket on the cone [muffler?] is further back, so you can't just slide a bolt in)
With the exhaust held up sorta where I wanted it, I turned the various 550 head pipes around and wiggled them up and down until they all were able to slide into the head.
I put the exhaust pipe joints and collars loosely on the pipes for 1 and 4 just to make sure the pipes would pull up into the head correctly.
Then I marked the pipes excessively with permanent marker.
All but one pipe stayed put on the trip to the welder.
Had a buddy TIG with stainless wire. He did a great job, and after he hit the welds with a wire brush, they were the nicest looking part of my whole system.
I bungee corded the completed pipes to the bike again, wiggled them into place with the joints on the pipes, put the collars on one cylinder at a time. To tighten the joints, I pushed in until I could start one of the nuts without the lockwasher. Once I had one nut on each cylinder, I went about tightening the nut a little, then tapping the other side of the joint to pull it up closer until I could fit the lockwasher and nut on the other side. Then I would remove the un-washered nut, washer it and tighten it down until it was even with the other side. I tightened each nut a little at a time and drew the "new" exhaust up into the crush gaskets. If I had been more patient and cautious in the mock-up phase, I may have been able to avoid doing all of this during the final assembly, but I figure I'll just tell you what worked for me. In the end, the "new" exhaust fit so well I was even able to reuse my copper crush gaskets without a leak. (I really wanted to see if I had pulled it off, and couldn't wait.)
To correct the rear bracket, I used a rear tire adjuster from a CB650, pointed the adjuster bolt down and back, put an "L" bracket on the bolt, "locked" it into place with nuts, and bolted the "L" bracket to the tab on the 650 silencer/muffler/cone.
Right now, the center stand pushes on the cone/silencer/muffler as I put it down (clears when up or down easily), so I am considering adjusting the "L" bracket to push the cone out away from the bike a little further. I just don't want to put strain on the head and cause warping over time when I could just hammer in a dimple on the cone to give me the inch or less of clearance I need. (I have not taken any pictures of this yet, as it is still a work in progress.)
Basically, this created a stepped or step up exhaust system like many aftermarket systems and allowed me to use what I had in the garage (plus a friend who is a good welder). I rented a chain tailpipe cutter from AutoZone, so I paid and then got my money back. Other than the very reasonable amount of money my friend wanted to do the welds, I spent $4.18 on hardware and it took me about 4-6 hours.
So, can a 650 exhaust fit a 550? Sorta
*Note: the bottom picture is from the mock-up stage; I added it at the last minute to show how I bungee corded the pipe up.
