Rust Mystery

[Franks 72]

Our truck had a similar problem (Canadian winters / road salt) . The patch repair worked well to keep out road dust and water however it masked the underlying decay.I chose to remove the entire crossmember which is at the transition from floor pan to plywood.This was nessesary to get at the rust in the vertical inner body panels which will need to be replaced. Your inner body panels dont look to bad but it is hard to se behind where he crossmember is bolted on.Check it carefully in this area. I have included a picture of the rust damage / patch.
Removing the crossmember is a huge job (spot welded / welded to floor pan) and I would not suggest it unless you have major rust issues. I would be interested if anyone has ever fabricated the inner body pieces since I will need to do this. Hope this helps.