You have to take the car completely apart because they’re screwed into the floor from the top.
If I need to shortcut it, unscrew the coupler from the truck, then carefully drill out the coupler mounting hole with progressively larger bits until it’s removed. That will get you enough space to mount the kadee pad.
taking the truck off the car allows you to either dremel it off, or ideally, disassemble the truck and remove the entire cage that the coupler is held on with. That’s not a job for the faint hearted, as you need to remove the springs in order to get the side frames off to get access to the cage screws behind the wheels, and it’s real easy to fire the springs off into the great unknown.
If you go through the trouble to get the MTH trucks off, you can also put Atlas trucks back on, which will lower the car a bit. Obviously it adds some expense, but the atlas trucks are a bit nicer as well. Theres a little button that comes with the atlas trucks in the package, so that you can mount them from the top like MTH does.