The front of the K1-K2 seat is "dished" inward on both the pan and the foam. This was done when the K0 tank's shape changed to the K1 type. The height of the tanks is not the same, nor is the curve where it approaches the seat. This causes interference if the wrong seat is used. You can "dish" in the front of the K0 seat pan about (a little more than) 1 cm, and then cut back the foam so as to match the curvature of the tank, but you will then have to re-pull the front of the seat cover, maybe even slit out 2 vee sections and sew it back together, to remove the wrinkle(s) that result. I have seen it done, and well, but it wasn't simple!
Also: the back end of the ducktail seat sits very high off the rear crossmember if the standard rubber block is used at the back of the seat. To solve this, I used the rear block form the F0/1 bike's seat back there (thinner) and slightly bent in the hinges so they did not stand as high and let the seat pan block(s) rest on the frame as intended.