Craig - these trucks were assembled by "and eye for fit and finish" - perfect alignment was never expected.
That seam has always been a source of rust. I have bead blasted, etchant primered, filler primered, sanded, sealer primered and top coated only to have it start a new bit of rust bubbles within 5 years if the truck spends any time at all outside in the rain.
If done as described above in your weather conditions, I bet you could get 10 years+ without problems.
