That sounds pretty similar to the panel adhesive I mentioned. I used the 3M brand one. It's used instead of welding on a lot of panels on modern cars. Apparently it's stronger than welding and not prone to rust.
For the roof ribs if you wanted a little more give in the adhesive (for expansion as Dyno Dave mentioned ) I would use a PU sealant like Tiger seal. That sets like hard PU rubber while the panel adhesive sets like plastic. And yeah replacing the ribs is the better way of doing it. Get a helper or two if you can ...sounds like it'd be the kinda job where you'd need to have a couple of extra hands.