I spent some time last night developing a new seat pan that fits the Pre 1974 frames for the xs 650. These pans are hand cut and formed, and fully TIG welded. Look for them in the shop!
Seats fit flat on the frame rail and follow the curve and shape of the frame rails. This means that they will look really good running a 'special' tank or aftermarket tank. As you can see from the picture the 'standard' tank will sit a little wide once the seat gets covered.