Door Rain Gutter on rear of 69 Dart - Rust issues

Hello folks, I really only have a few rust issues to deal with on my 69 but I wish they were as simple as "replacing ALL the floorboards". That probably sounds humorous to some of you but I have learned that once you learn that it's pretty easy and straightforward.
RightRearDoorGutter.jpg
What is above is the rain gutter above the passenger side door. If you look close corrosion has caused the root panel to seperate from the drain gutter as the rust swelled up and pushed them apart.

I suppose the absolute correct way to repair this is to cut the vertical piece of the roof and separate the piece pinched in the other portion of the gutter and weld it all back together after soaking jt all jn Rust-Mort. That's probably good for a week's work (for me) and then I would still be unhappy with jt.

What if..

...I were to spray the crap out of it with Rust-Mort and then fill jt with POR-15 putty. Then I would use POR-Patch to make a smooth waterproof bond that, (in -theory) will never leak, and,in-reality will not leak in my children's lifetimes.

por_products.jpg

Your opi ions please O'Wise One's?