Hi Chuck,
Nice to see you made it over to this place.
I believe that the plotter gantry was animated by means of a long toothed belt. You can make this stronger by removing this and running the gantry with a screw and nut system. This should make it alot stronger. You may also need to find stronger stepper motors, or maybe the existing ones can be sent more DC voltage to make them stronger. You can go as much a 4X higher than listed on the motor. You can NOT exceed the rated Amps.