A question: Is the roof and or the floor glued to the car-sides? If so, the easiest way to add trucks will be to un-glue either the floor or roof. If un-glueing the floor or roof isn't an option, you may be able to get away with a small barrel nut glued to the bolsters.
I'm not a fan of the Atlas commonwealth trucks. The pair I have don't fit well at all. The coupler shank is too short which moves the trucks to close to the ends and interfere with the steps. Not to mention it looks bad. I use MTH premiere 2-rail 60 ft. woodside passenger car trucks. The 2-rail wheels work fine on my layout, but you may have to swap the wheels out for high rail wheels. If you want to light the car, 3-rail pickups are available from MTH.