'64-'66 Abody Cowl-How did Factory Make it Water Tight?

I think someone who was doing the same repair you’re doing suggested an adhesive panel bond.
Panel adhesive would work well. You would have to weld up drilled spot weld holes in your donor cowl. Use a lot
of clamps with the panel adhesive and weld the bottom windshield flange. The ends of the upper cowl are welded
to the bottoms of the A pillars. Those seams were filled with lead from the factory. Unless you're familiar using
lead, use a fiberglass product, like Kitty Hair to fill in those seams. Once the seams are filled with fiberglass, finish
the area with plastic body filler like you would any other body work.