I have/had one on my 550, can be seen in my avatar and gallery
http://www.sohc4.us/gallery/main.php?g2_itemId=13230. They're not bad to fit, but they are pretty generic. The seat fits over the frame rails (not on top), but is too narrow to use the latch mechanism. I'm not sure about the fender, I didn't run one with it, but I'm almost positive it would hit the stock taillight. It might work without the taillight...
I originally used the rubber bumpers off of the stock pan and drilled holes in the fiberglass to use the front pads (the curved ones) at the front and then punched holes in the two round bumpers and ran bolts through existing holes in the flat cross member, worked pretty well to hold it in place, but no hinge for easy access.
I ended up cutting the front pan section off of mine and just using the "bump", and made an aluminum pan that followed the frame rails more closely (the fiberglass pan doesn't taper, the rails do), which is what's in most of my pictures. I'm in the process of fitting another seat that sits on top of the rails, but thats my preference...hope this helps!