Yesterday I drove upstate to see my parts source. I bought a very good condition inner heater box ($10) and two amazingly rust-free inner fenders for the 67 GMC ($40).
Here is my old heater box. It was crushed on one end, but I am not sure how that happened. And it was full of rust (no surprise).
Here is the newer one. It has the heater switch box that I recently restored attached to it in this picture.
Newer one installed.
Rust-free inner fenders. My old ones were worse than I thought after I removed them.