Well, Most bodymen would use a product like "3M Body Seam Sealer". It's flexible, paintable and comes both in a tube (like weatherstrip adhesive) or brushable from a quart can. May be available in pints too, but for what you are asking, the tube should be sufficient. I'll be using it on mine when ready.
A specific seam sealer is available @ any auto parts store that carries autobody supplies.
if you are not particularly a steady hand at operating a caulking gun, or don't have a power caulking gun available to you I recommend masking tape on both sides of the seam, apply the sealer and using a rag that is well soaked with solvent wipe over the freshly applied sealer to smooth it and help push in to the crack. Then proceed to remove the tape right away whilst everything is still wet so you dont get any ridges on the seam sealer.
Oh ya.... by the way make sure that the inside of the seam is nice & clean, free of rust and primed before applying the sealer. Finish over the sealer with whatever primer you are using and you are ready for your topcoat.
Don't forget to check how the factory seam sealer islookin' inside the drip rails, with it being over 50 yrs. old it's likely pretty dried & cracked, you probably should freshen this area up whilst you are at it. As well as the seam between your boxside & taillight panel if it's a styleside box.
Mikie near ottawa canada
58 M-100 panel & 64 Econoline p/u