Lead might be an option,alternatively there are good flexible,paintable seam sealers available.
Most modern quality body fillers are to some degree pretty flexible too. I don't think there should be a lot of movement in that area of the cab anyway.
1950 Chevy Advance Design 3100 in Scotland In the Stovebolt Gallery More pix on Flickr. I've definately got this truck thing in my blood ... my DNA sequence has torque settings "Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.
