Not quite sure where you are describing, but I will take a stab at it. The undercoating was often done by the dealer and could be most anywhere. My suburban has it on the lower portion of the fire wall and inside the wheel wells too.
Is the portion of the cowl that you are referring to just above the louvers on the right side and proportionally equal on the left side? If so, that is a factory seam and yes it will have been leaded at the factory. You will also find a leaded seam on the roof pillars adjacent to the front windshield about the top of the vent windows or about three quarters the way up from the dash on the outside pillar..
