The problem you have with clearing your original paint is that the surface will have to be sanded/scuffed over every square inch to give the clear something to adhere to. Without sanding it is just a matter of time before the clear delaminates from the surface due to poor adhesion and just peels off.
Also, please don't shoot rattle can clear on your truck. It is just unhardened enamel paint that will just have to be stripped right back off when you repaint. Your truck will be OK driving in its original patina, just make sure if you have any sizeable bare metal spot that maybe you lightly dab them with some matching color so as to keep them from getting rust pits over the next few years.
