Let me see if I can offer more details that might help you.
First, the firewalls are different between the AC and non AC trucks.
What I did first was to remove the old AC heat system inside and out. It's entirely self contained except for a single brown wire that connects into the main truck harness behind the control head on the dash. There's a plug right there, just unplug it.
Then, I had the benefit of having an old section of firewall from a non AC truck. I initially figured I'd just splice it into my firewall, but it was rusty and warped and not really suitable. So, what I did was lay the old firewall piece on a piece of flat steel and trace the holes with a marker. Then for the outside edge, I made sure and make it enough oversize to cover unneeded holes from the old AC system.
The holes drilled in the new plate are spot weld locations.
I could have absolutely cut and fitted the new piece in place and butt welded it and slicked it out to look GM original, but for some reason, I just don't care. So I cut it oversize and spot welded it right over the top of the old firewall holes. I just made a cover plate like a Vintage Air system would supply or whatever.
Masked and painted with close match of rattle can. I did apply sealer around the edges of the plate before welding it in place, then ran a nice smooth bead around the edge afterward. I figure this truck not having fenderwells, weatherproof was paramount.
Then install the inner box and out box. I used a new heater core from O Reilly Auto Parts. There needs to be some form of seal between the black box and the firewall too. Originally it was some sort of strip caulk. I was going to go buy a new box of strip caulk till I saw it runs nearly thirty dollars. So I went to Walmart and bought a new tube of black silicone for my caulk gun instead. I just laid a fat bead of the black caulk in the groove around the outside box and bolted it in place. Ghetto, yes it is. But, there's no reason to ever have to remove it, so I figured sealed from leaking is sealed from leaking.
Then install the wiring and cables and control head. The wiring passes through the same hole in the firewall as the original AC wiring did. It all falls into place. Again, the heater harness is completely self contained as the AC setup was. All you have to do is plug in the brown wire into the truck wiring harness under the dash. No cutting or mods needed. The heater box fits exactly onto the defroster plenum as the AC box did. No mods or reason to change the defroster ducting. It's plug and play.
I did this because I don't have the scratch to spare to buy an aftermarket AC setup. And, you can drive a truck with no AC, but not one without a heater. When the weather is bad and the windows are icing up, you must have a defroster. This mod opened up a bunch of engine compartment room and hopefully tire clearance. The old AC system didn't work anyway. What I have found is that heat only systems blow much much harder than heat AC systems do. When I got finished with this swap, I turned the heater blower to high and slid the control to defrost and it blew a glove I had laying on the dash off onto the floor. It cranks the volume much better than the AC system ever did.
I don't feel much bond with this truck so my mentality on this build up is to get it done without spending undue time or money on little details. Like buying strip caulk when silicone from the caulk gun will do the same thing for 25 dollars less. And spot welding the new plate right over the firewall rather than getting into panel fitment and body work.
The last vehicle I built with this mentality was my 54 Chevy. I just cut aluminum block off panels for the firewall, screwed them in place and brush painted the entire thing with Rustoleum red oxide primer. I had more fun with that car than any other and it stopped traffic every time I drove it. Folks went crazy over it.
I had that car, all stock and bought a 78 Impala as a parts car, and took a week off work and built it. Mustang 2 front suspension, engine, trans, wiring, everything in a week. Man it was fun too. Ghetto sh1t, digging in coffee cans for screws and bolts, cutting up parts car wiring to get the right color and wire gauges I needed, and then I drove it daily for years. I drove that car 365 days per year all through EMT school, Paramedic school, Fire Academy, and through my rookie year here at the department. Something like 80k miles total. I had quit my job with the Post Office and was going to schools for a career change, so it was sink or swim. I had no income and driving something with a payment wasn't an option. It was a legit 18 miles per gallon car. 305 engine, 2 barrel, TH200 trans and 2.73 rearend. The stance and overall vibe is what made it so neat, not details and fit and finish. Underneath it was lackluster like this truck will wind up being. But, at 70 miles per hour in the fast lane, folks don't notice if you fussed over the details.
Cliff notes, I'm trying to have some good dirty fun here and not go all mental over details as y'all have seen me do on recent builds.