[DetroitDan]

I just bought some undercoating in spray cans when I replaced my bed. New bed still had all factory paint underneath it, sprayed it down with about three cans, it dried and flaked right off. Waste of time and money. Must have bought the wrong brand, I've used stuff before that was more rubbery and stayed kind of gooey.
Personally I'm not a fan of commercial undercoating, I've never seen anything but rotted out trucks with Ziebart or Rusty jones decals. I know it supposedly works better on new vehicles but I've heard not to do it to an older vehicle because you are just going to seal in the salt particles and any moisture thats already had a chance to get in.

Most old-timers I know swear by oiling the underside. I do mine that way once or twice a year, I save my waste oil (diesel is nice because it stains black and looks painted) and spray it with my cheapo Walmart sandblaster. Only takes a few minutes to do, park it on some big cardboard pieces and let it drip dry after. Free and it works the best imo.

Have you considered doing it with linex? They sell DIY kits that roll on, I would do that if I were doing a cab-off. Whatever you do, make sure it's clean and dry.