Left for Dead 68 Barracuda Buildup

I came across my first rust issue that is more serious than just removing the surface rust.
It appears it has rusted from the inside out in this area. Didn't
make it self known till the blasting grit found the weakness.
This is the area that the air conditioner unit bolts to the inside firewall. I will be removing the factory air unit and leaving it out of the build. The original Hurst cars had a block off
disc to fill the blower motor hole. Heater delete so no heater box will be installed.
I knew I was going to have to do something in this area due
to the firewall being hole'd for factory air so the rust thru didn't
really change anything.
The big question is do I remove the inner fenders and replace
the entire firewall with the correct non ac part at the factory seams. Or do I splice in a section from a non Ac car.
If anyone cares to give their opinion on this repair decision I would like to hear what you would do. I have a few more days of blasting to do so I am not rushed on making up my mind.
This car has many cut and splices around the car already
so I am leaning in that direction.