Inside door treatment - 1967 GT350

You can use gasket material to make another insulator (with a little contact or weatherstrip adhesive.

See others just apply a bead of epoxy or seam sealer

 

I like the wurth sprayable seam sealer, works great for sound proffing. I also spray some Crown rust proofing in the rattle can with a long tip nozzle along the seams after all the paint work is done. also do the rest of the 1/4's on the car the same way. It'll drip for a bit, but it creeps into places paint and primers can't get to.

 

For rust prevention, I use Amber Rust Proofing from Eastwood. I recall that 3M also make a similar product. It's like a syrup, so, it sticks but also will run down into the cracks where rust starts.
On a Mustang, the main areas in the doors are the front and the rear corners.
The front especially is triangulated and hard to get into with anything.