I filed the Kadee coupler sides enough to fit the box that was there. I used the spring but not the Kadee box. The coupler will not self center but I can live with that.
Rod Miller also had a way of doing this on Max Gray and U.S.Hobbies cars. Rod's text and images are below.
Here is the info on the KD coupler adapters for Max Gray and U.S.Hobbies cars.
The first photo show a KD in the original MG coupler pocket.
The second is how to make them: a piece of .020" x .25" styrene bonded to a piece of 1/8" x 1/8" styrene. Length isn't critical, just so it fits into the hollow in the coupler (you have to force it in) - the adapter in the photo is .3" long.
Put the coupler in the coupler pocket and mark through the hole in the bottom of the pocket where to drill the hole for the mounting screw.
My last car, a MG 4 truck flat car required some minor filing and fitting, e.g., YMMV but still simpler than removing the coupler pocket in order to install a KD box. There may be little coupler swing but the slop in the knuckle gives plenty of leeway, and no automatic centering of the coupler - hey, there aren't centering springs on the real thing!