Under drip rail rust repair

[mm1077]

My new truck is going to be a driver but I’m sorting out some of the worst rust issues in the cab right now. The area of the door jam directly under the drip rails are rusted through and I’m wondering if I can safely cut out a small section of the roof to access and weld in a new piece from above or if I can just patch in a section from underneath the the drip rail? The drip rail seems to be pretty solid but ideally Id like to open it up and get some rust converter over all of it, but not if it’s going to turn into a major ordeal. The windshield channel, A pillars, etc aren’t showing any signs of major rust damage from the interior or exterior.
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