One way, though possibly requiring a bit more work, is to remove the truck-mounted coupler and pocket altogether and install a body-mounted coupler pocket and coupler. On a locomotive or 40-50 ft. long freight car, that is a much better way to mount the couplers, as truck-mounted couplers tend to derail when pushed. Basically, you remove the truck mount and drill a hole for a #2-56 screw in the underside of the car body. You may also want to cement the Kaydee coupler pocket to the floor as well as using the screw.
The other way, if you want to keep the truck-mounted is to modify the existing coupler pocket, probably drilling a screw hole and/or using appropriate cement (CA, styrene, etc....whatever works).
On 85 ft. long passenger cars you may need to use bolster extensions, also sold by Kaydee for AHM and others, in order to have enough swing going around sharp curves (18" radius or even 22" radius), or just swap the couplers on older Athearn and Con Cor cars.