There are two spots on either side of the truck at the cowl area where body putty has been applied. Normally the problem is the seam that starts at the corner of the windshield and goes down into the firewall. I have seen the seam sealer at the roof rail crack and leak all the way down through the windshield buck and into the floor.
Exactly this.....That seam at the lower corner of the w/shield has been known to crack completely down to the pillar...especially on 4 wheel drives that see a bit of body twisting. ALL my '74 to '80 trucks I owned had cracked and ALL had water leaks onto the floor. You might have to pull back the fender at the top and re-weld the seam and caulk it.
