65 Dart Fresh Air Vent Area Rust

Yep, windshield and dashboard have to come out. Top cowl and lower cowl need to be cut out from the firewall and pillar posts. Your lower cowl is rusted and looks like you need to replace it. Upper and lower cowl are sandwiched together with the firewall and spot welded. There is seam sealer between the firewall and lower cowl, maybe between the upper and lower cowl as well. Spot welds need to be drilled out with a spot weld cutter. Both joints at the pillar arm are leaded, remove this first, then spot welds drilled out and there will be mig welding as well that needs to be ground away. Start looking for a replacement lower cowl now. I believe this is going to be the hardest part. Also your pillars will be need to be supported by welding braces between door openings, windshield opening and the floor before upper and lower cowl removal. More than likely you will need to remove headliner and interior trim. Reassmbly must be spot on or you risk having a leaky windshield from the opening not conforming to the windshield.