It's a really long seam across the back of the cab above the window, I'd be afraid of the shrinkage from such a long weld, it's a good sized gap there.
The lead at the rear of the door openings, right at the end of the rain gutter is super thick, like 1/2 inch.
A guy on the stude forum who does a lot of painting said to leave the lead and epoxy primer over it and then use the fiberglass reinforced filler.