Chrysler built 750,000 440s and who knows how many 413s. I have a hard time believing that someone would weld up two journals on a 440 crank. That'd have to cost more than the core charge for a different crank.

Crankshaft welding is done by the submerged arc process and it produces a very good result. Heck, any weld is expected to not have porosity. It is not, as some posters imply, inevitable that a weld will have voids.
Any certified welder should be able to produce a weld with no voids using a flux coated welding rod, commonly called stick welding.
I mean really, I am far from certified but my stick weld samples passed guided bend tests. Chip off the slag and get enough penetration on the next pass to melt out any unseen slag inclusions.

My other clue is that the pits occurred in a line and not spread randomly over the surface.

My best guess is some sort of acidic material attack on the uncovered parts of the crank journal. I have seen pitting like that in an engine that blew a head gasket and got antifreeze in the cylinders, where it turned into something acidic, possibly glycolic acid. Boring the cylinders 1/2 mm, or 0.020", didn't get to the bottom of the pits, but it got close. The dark spots at the bottom of the pits were still visible after boring.

R.