65 Dart Fresh Air Vent Area Rust

On my 65 Dart right where the driver side fresh vent attaches to the car is completely rusted out, so much that when I drove over some railroad tracks the vent fell of and rust and leaves blew inside, question is how hard is that to get to? I'm stumped on how to get to it as this is the only major rust spot on the car. The attached image shows the rust area right next to the brake pedal and alongside the firewall.

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