[dsraven]

yeah, kinda what I was thinking as well. don't do it unless you have to. what about a garden sprayer with some undercoating so you can pressure spray the entire void to get to any cracks. let the excess run out a drain hole in the bottom (placed there on purpose to let out any moisture or condensation that may show up in the void) then cap it with a removable floor cover piece on top. it could be sealed for noise with some seam sealer but remain removable and not attached to the outside skin.
on another topic, be careful how you support the cab on stands. the running boards are pretty strong but can distort the rear cab corner if you end up sitting on the cab while repairing and the cab corner is a little suspect anyway. best to have a door hung in place and latched so you get the cab corner to door gap correct. maybe best to support the cab under the actual cab support areas to be safe. when I did my cab work I cross braced the cab with the doors on so it couldn't move., it was also siting on the truck frame so the contact points would also remain the same and not distort later when the cab is bolted down tight.
your truck though, do it like you feel you wanna.