A couple of things....

You only have to remove the inside half of the box. The blower in the engine bay can remain moutned to the firewall.

The NPD heater core is not the correct thickness. I used it only to get me through until I put the vintage air unit in.

Also, you can get 2 of the 3 doors in the box out without drilling the spot welds. You can drive the hinge pins out (That's where the heat and pursuasion comes in). The third door is reachable still in the box, if you don't have really big hands (please hold all jokes). I used heat to disenegrate the old seal, and then just put the glue on the seal, reached my hand in the box (again, hold the jokes) and replaced the seal.