I recall an old article by Bob Adler (a true expert) that suggested cleaning the seam out as much as possible; then drip a
rust converter into the seam and let it dry out; then drip/spray epoxy primer into the seam. Then, proceed as described above by Tim (SW_Radial).
If that seam had been "sealed" with lead, the leading was not done where you can see it on the outside of the cab.
I did not have to deal with this on my Suburban (no seam) but will soon be dealing with this with a 54 pickup.
Tim
(rust converters similar to the one in the link above are available at NAPA and just about any hardware store - Rustoleum makes one that is readily available)
