My '60 has about the same problem, here is what were are going to do.
1. using a sand blaster remover the lose rust from between the roof and the cab top.
2. Weld up the rot/holes/edges.
3. The use por 15 to coat the inside of both parts, it it cannot be sprayed, we will use a schultz gun to spray in undercoating.
You can reach the recesses via the "two" vet area in each rear corner.
