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.

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