A long time ago, Kadee used to make a separate coupler to drop into the the truck-mounted coupler boxes - I believe it was a #9. However, the #5 was identical except for coming with its own draft gear, so #9 was dropped.
The #5 and centering spring plate will drop right into many draft gear boxes, both body mount and truck mount. That's probably the 1st thing to try, and the cheapest way to get Kadee couplers installed.
On truck-mounted couplers, there is a real tendency for the weight of the draft gear stuck out on that plastic arm to cause the coupler to sag, which causes the trip pin to catch on trackwork and couplers to under-ride on vertical irregularities and uncouple. Also, cars with truck-mounted couplers do not back well. If there is any side push from the coupler, it is magnified to the wheels through the long lever of the mount arm, and causes the wheels to look for and derail at the slightest hint of a track or wheel problem. Most cars with truck-mounted couplers use cheap plastic wheel sets, which can be out of gauge, out of round, or not properly mounted on the axles.
For all these reasons, often the best upgrade for these cheap cars is to replace the trucks and install body-mounted Kadee couplers. Doing those 2 things will definitely increase your operational reliability significantly. But it does cost $4-$5 per car for new trucks and couplers.
Just body-mounting the #5 couplers and cutting off the coupler mount arm on the trucks will give improvement, especially in backing up your train for switching. The biggest issue in body-mounting couplers is getting the coupler height right. You WILL need the Kadee coupler height gauge regardless of which options you choose. If the gauge shows the coupler low (not likely on a switch to body mount), you can add a washer above the truck where it mounts to the body, file a notch in the underframe for the draft gear box, or switch to an overset Kadee coupler (shank is at the bottom of the coupler knuckle). If the coupler is too high, you can shim the draft gear box downward. Once the coupler knuckle is at the right height, you then adjust the trip to the correct height.
Many modelers like the centering whiskers of the new, otherwise identical #148 couplers much better than the brass/copper centering plate of the #5 coupler. The centering action of the spring plate was not always the most reliable for me. Often I found smoothing the front edge of the plate and the shank of the coupler with a file helped. Also, I needed to make sure the swiveling of the coupler on the pivot pin was smooth.
yours in coupling