Climberman, I have a 1966 C10 as well. My cab has a very small spot trying to develop in the cab over the drivers head. I also have a 1966 C10 donor truck and it has severe rust through over the driver and passengers head, right above the windshield.
I was stumped as to why this was rusting because there are no holes in he roof. I finally realized that the seam sealer in the drip rail has broken down over the time and is allowing water to seap in. I believe the inside could be repaired and then clean and reseal the drip rail. I am going to cut into the donor cab to inspect some more and I will keep you posted.
