Guys Thanks for ALL the inputs but Holy cow lets not argue over my seam.
Henry Ford put these roofs together with spot welds and then rolled the lip on to the top part of the roof from the cab section and that was what made it water tight, as far as Ford was concerned or as I can see from taking this one a part. My whole reason to lead it in was to produce a weather tight seal along with a nice looking fillet at that seam. I do have a 2 part self leveling seam sealer that is used for auto body repair sitting on my work bench.
Seam Sealer by 3M - Sealers by Zoro Tools Industrial Supplies

The lead work the big car company's did was mostly to join quarter panels and such together and they didn't have any rolled lips to in-trap any acid flux ect. I've taken quarters off a camaro, the quarter and roof panels were laid one over the other and spot welded , and leaded over. There were no lips so everything could be cleaned off inside and out .

I think the biggest thing is just to keep water from seeping in to the seam and causing rust.
Maybe I should just RATROD this thing and go smoke some tires !