1966 C10 Cowl Repair

[Afirehawk1]

Need some advise on this. I removed the complete inner cowl panels on both sides to expose the underside. This is where all the debris would have built up. Some one in the past went under the dash for the repairs. Trying to permanantly and correctly fix this. Obviously there are no repair panels for this. The entire cab has also been media blasted. I was considering cutting out the rotted metal since there is now good metal from underneath. Would it be practical to use Metal 2 Metal fill or maybe Tiger hair? I want to permanantly waterproof and seal this area for good. Or do i need to add new metal? Then seal?

Also i have to replace the water divertors on both sides.

Any experience on this. Thanks!