OBS: possible. Requires pedal box, master cyl for clutch, harness, trans, flywheel, steering column, and ecm flash. The trans hump works fine, as the floor pans are the same.
Newer trucks 99+:
For the manual truck, the floor pan is taller than the auto, so you are fine there. You'll need similar year trucks, a new or flashed pcm, auto trans, different transfer case (or input shaft), flexplate, new master cylinder, wiring harness, torque converter, and a new steering column. If you were going auto to manual, your floor pan needs to be modified. (read: $$) Or you could cut a giant hole in the floor and have the shifter stick through...:no: < ******* rig that sucker. If it were me, I'd leave both shifters.
EDIT: I thought this question was familiar HAHA