The latch is OK, but the hinge tabs on the frames are a little different, requiring you to move the hinge piece on the 400F seat pan forward about 1/2" so. Not a huge deal, since the pan is pretty flat in that area. You could use the 400F tank on the 350F frame, but the OEM 350F headlight mount ears will crunch into the tank at full lock (the 400F used bare fork tubes). Surely an OEM 350F seat pan will show up for you, but if you get your hands on a 400F seat and want to continue using the stock 350F tank, I believe I would remove the upholstery, fab an extension for the front of the pan to fill the gap properly, add foam as needed, rework the rear of the pan an appropriate amount and reinstall custom upholstery.