[67ChevyRedneck]

In your area cab rot above the windshield and around the drip rails is common on these trucks when they spend a lot of time in rain/near the ocean.

Literally sit inside the truck and pull the windshield rubber down around the top of the windshield and inspect the drip rails thoroughly. Most rust repair on these trucks can be done by a novice with patience and practice, the roof area is lot more difficult to tackle. Gently push on the metal above the windshield too, if it moves and makes crunchy sounds it's probably bad.