Not really. After just trying to put a repro 400F seat on a 350F, the hinge is different and the lock plunger is mounted in a different position. Even if you have a 400F or similar hinge, the lock doesn't line up. The pin on the 400F hinge is also not long enough for the 350F mount, which uses humongous cotter pins instead of a long pin. Obviously it could be done, but not a straight swap.
Also, the grab bar interferes with the 400F seat and would need to be removed. The turn signals would need to be relocated. Rear signals on a 350F use a bolt on rubber mount type stem, not a straight stud type stem. The stud type stems are too large for the mounting hole from the grab bar, so you'd need to drill out the hole in both the frame and fender to mount a stud type signal stem in that location.
