I Tac welded the seam around the back with my wire feed , and am going to use Silicon Bronze Rod and my Tig, and fill the seam ( & grind smooth ), as The Silicon Bronze melts at a lower temp so It wont warp the panels. Silicon Bronze is what the factories use to seam the 1/4 panel to roof joints on new vehicles.
Body Filler , primer , stick real good to it , unlike brass .

The drip rails were in sad shape , bent, smashed,& rusted , so I used my air hack saw and cut them off, and got 1/4" steel rod and welded it in the same place as the old drip rails, and then ground the front & rear ends to a taper to flow into the rear roof line & cowl.

I would try and keep the drip rails on your rod if they are in good shape ! I would reccomend sand blasting down in the channel to get all the old seam sealer and crud out of there. Then seal the bare metal with something like PPG's DPLF 90 , let dry then get an automotive paintable seam sealer and run a bead of it inside the channel, then smooth out as best you can.( you can use PPG's DX 330 wax & grease remover to help smooth it out as it really works nice.)
Let it all set and dry for a few days then prime it and sand smooth.

Spray