There is a better picture of this area on thread
https://www.forfmjbodiesonly.com/classicmopar/threads/heater-box-removal-a-c-1980-f-body.6464/#post-72252 post #12.
This picture is from that post.
What you see is the firewall. There is another part that goes on top of that for the Cowl (which hides the wiper linkage).
On the Left side of picture (Right side of car) there is a 1 inch high lip that is “D” shaped over a large hole. That hole is for the main ventilation system for outside air.
On vehicles with Left side outside air vents (no A/C cars), there is a similar system on driver’s side and an air box that bolts from underneath the dash.
Generally, people want to install the Left side vent on cars (like myself).
The area between the firewall and cowl sees a lot of water (when it rains) – which is why the 1” high lip present.
I “think” a person needs to find an older heater-only case and cable, like out of an F-body, and I would assume it should bolt right in place.
Honestly, I hadn’t seen very many no A/C M-bodies to look at, at least not in Oklahoma - so not 100% sure on interchange - but I do know that '76-89 F & M body no A/C firewalls are all the same.
With that said, I happen to have drove my no A/C F-body to work today (93’ F).
BudW