You can see the two nuts holding the heater assembly to the firewall on the engine side. You can also see the two studs that have nuts on the inside.
I have read that one can reach the nuts without removing the AC but haven't tried.
But maybe you can separate the two halves of the heater box without removing it from the firewall?
Sic friatur crustulum
'84 GW with Nissan SD33T, early Chev NV4500, 300, narrowed Ford reverse 44, narrowed Ford 60, SOA/reversed shackle in fornt, lowered mount/flipped shackle in rear.