Hello,
Your air handler looks about the same as mine did. I took the entire heater box out of the car to repair the plenum. Everything is easy to unbolt if the fasteners are not rusted fast. Everything but the connection directly under the cowl can be removed. Hence my problem. The rectangular box is very rusty and needs to be replaced. From what I see on the outside, the rectangular frame is spot welded in place. I am going to sandblast the area to find the spot welds and drill them out. Then I can fab a new box and re-weld in place.
Instead of making the entire plenum, I fabricated a lower section of 18ga steel and fit it to the top section. It was a bit time consuming and is far from perfect, but it does the job. At present, I have not seen any aftermarket replacement plenums for sale or the part welded under the cowl. If you find a source, please let me know.
Thank you,
Donny-b