How to remove outer wheelhouse?

I also wouldn't be concerned about sealing the tub, use a high quality seam sealer inside the tub where the flanges all meet and you won't have any issues. I don't believe that you will get any moisture from the outside of the tub between the flanges since it's all buried inside the car and never really gets wet in there. The only rust I had was the kind you would expect from bare steel being exposed to the air for 43 years.