I tried an early 500 tank, way too much work, the 400 fits much closer. The only real interference is the two lower edges at the rear of tank, either side of mounting tab.
I tried a seat or two. I think the flattrack one cut off just forward of the shock with proper seat base, would look really good but needs a hoop bent to go under outer edge of fender. At the front it would have a downward bend lining up with the diagonal frame member that runs from shock to motor mount area.