Depending how far you want to go and to what measures you want to go to, one of the things I've looked at doing to my early 65 D100 is lightening the frame rails. The rails are really tall "U" channels that lie on their side and the steel used is heavy gage. It would be really easy to make circle templates (make several in different diameters) and then get under the truck with a plasma gun and the tracing templates and start removing steel. You would want to plan this out ahead of time so the areas you want to cut are laid out or marked on the frame as well as understanding what other pieces are in the area you need to be careful with. If you're working on a driveable truck, you would need a few fire retardant "welding" blankets to protect adjacent parts, wires, hoses, tires, etc. It's certainly do-able with care taken and not rushing the job and this can easily be done without impacting frame stiffness and safety.

- EM