Need rust help..

[usetaboost]

Started undoing the body bolts today. Nice and easy, hell most of them are still good and came undone real easy and the rr was near loose! The ones that are in front of the rear end there is a support tacked to the floor that goes from one side to the other. Well I got to the passenger side and it broke apart where the mount sits cause it was all cancered out. My question is can I just replace this support that goes from one side to the other? I would guess just tap the welds with a air chisel or something and it should drop, right? What should I use to get the rust off the rest of the floor, it all looks like surface, feels solid still. How high up do I have to get the body up to clean the underneath of the body and protect it. Kinda doing a half ass frame off. Thanks to all the pro body guys out there!