after you pick up the Kadee height gauge, tape an assembled Kadee coupler & box to the underside and check the height with the gauge. without seeing a photo of the end sill, it looks like there's a bit of draft box molded into the end.
once you have a reference point with the gauge and temp-mounted coupler, you can adjust the body height up with washer(s) between the truck and bolster... or down with coupler pad shims.
either way you should drill mount holes in the underbody
