Upper Cowl Rust

[Blue666(2)]

I have finally located the source for the water leak behind the dashboard. I checked from below and it does not appear to extend any higher. I removed the vent cowl and saw that the firewall is seperated cowl panels.

Has any one else tackled this problem. How much firewall do I remove so I can clean and reinstall? Will the windshield need to come out?

Also, can anyone recommend a better spot weld cutter? I drilled about 20 spot welds and these were toast.