Cracks around metal at base of windshield

George Jets said

Might be flexing from the driver opening the door and it getting caught in the wind flexing the hinge pillar and stress flexing the lower windshield frame.

I would say hang onto those doors when you are opening them, and it probably won't go any farther.

Those are called battle scars from it coming through 50 years of time . . .

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There is a leaded seam at the outer parts of the upper cowl and the windshield pillars. The lead won't rust, but the metal underneath can.
There maybe water infiltration at the windshield pinch welds causing rust and cracking. Removing the windshield and gasket would be required
to really see what's going on. The lead is also on the vertical edge of the cowl from factory metal finishing, that can be seen when the door is open.
That is my take anyway.