There is no "correct" answer, but I do have questions about what you are doing and why. Painting involves a lot of hard work. To get a good paint job, the old paint must have good adhesion to the metal, no rust underneath, and it must be stable (not breaking down especially to UV rays). Unless the paint is in good condition and there is no rust beneath it (not likely on old trucks) I would recommend removing the paint completely, either by chemical (stripping) or mechanical (sanding, blasting) methods. Add to that a coat of good primer (I prefer an epoxy) and you will be on your way to a long future with your new paint job.
Now if you are trying to sell the vehicle, it's an entirely different story.
As for sanding, many use an orbital sander first going from a medium to a fine paper. In-line sanders are a great help, too, but sanding with rubber backing blocks by hand is tough to beat, especially for the final smoothing. You might want to post this question in the paint & body forum, there are some there who are far more knowledgable than I.
