Many, or perhaps even most, Airstreams of this vintage need some floor replacement and frame repair/rebuilding. My frame had two outriggers just aft of the streetside wheel well that were badly rusted, one of them was actually completely broken off and just sliding around.
I'd expect that you're in for some floor and frame repair, first fix the leaks that are the likely cause.
Your year model is fairly similar to mine, and on mine the c-channel is actually a u-channel that sits on top of the plywood subfloor. It bolts through the subfloor into the frame members and outriggers, and also has some wood screws throught it directly into the plywood. The belly pan wraps up from the bottom and is also riveted into the c-channel, with the outer skins overlapping them to shed water.
To get to all of this properly, you'll most likely need to cut away some of the bellypan to inspect, or at least pull the rivets and drop it to inspect. If you see damagae, you can cut away the bellypan to access from below, and remove the lower interior skins to access from above. If you've done a Spartan, then you know what this involves, it should be similar.