Francine, I have used the method that Carl Tuveson describes on his page....http://www.tuveson.com, then "hints and tips", then "changing AF waffle boxcar trucks..."
However, a far easier method is to fully replace the chassis with an Ace floor. And Doug Peck at Portlines has just recently added some Ace floor reproductions to his stock. You just screw the boxcar shell to the Ace floor and then screw your AM, SHS, MTH, DPH, or Ace trucks on. I usually use AM trucks and truck screws for this.