500s and 550K1 take the same pipes. I didn't check any later 550s, but they may also be the same. As for your carbs, they will come off after the airbox, cables, fuel line is disconnected, and the intake boots are loosened. They will come off in a single rack. You can do virtually all of a carb overhaul without seperating the carbs from each other. This will keep some sense of syncronicity of the carbs. But before you remove the carbs, check the cables for movement now that they are disconnected. This may be your problem.
http://www.hondaparts-direct.com is what I use for factory parts info. They have factory Honda parts fiche online and will have the most current Honda part numbers. Hit the button on the left "Honda parts finder" and fill in the bike info. I went to '72 and '73 and '74 550K to answer your exhaust question. I have never bought from them, I use my local dealer, as I am a former employee and still get a "deal".