I believe Napa carries a 3M brand seam sealer. Honestly it is the best stuff out there. It stays semi-soft but is sandable etc. I have used it extensively on my 80 Camaro to seal the new floors I welded in. One tube goes a long way, so I would have no idea how much you would need for your bed. I would think 3-5 tubes depending on what you are doing. The stuff is expensive though, just keep that in mind. Something to keep in mind as well: if you intend to POR-15 or KBS coat the bed, then DO THAT FIRST. Once it dries, then seam seal over it. This should give you a well-sealed and protected coating. I just did this very thing on the floors of my C30, turned out great. Good luck!
