How much PSI on the a/c system???

Nope. In the Bronco II (I guess this was a common Ford design for a certain period), the HVAC enclosure was a two-part plastic case, one each side of the firewall. The evaporator was on the engine-compartment side (so was the blower).

If you had another pair of hands, you could unbolt the compressor, condenser, a few misc. hold-downs, and the evaporator, and take the whole system out of the truck without opening or evacuating. (The main engine wiring harness went "through" the loop, the only real thing in the way. Had to be disconnected from the ECM inside and the grommet pushed through the firewall.) I actually took it that way, in the back of a friend's car, to the dealer to have the R12 reclaimed. They hadn't seen that before.

It would have been possible to do the whole enzyme soak thing without unsealing it, if I hadn't wanted to preventively replace some rusty-looking tubes and some O rings while I was at it.

The heater core was an equally easy job on that one. It was on the in-cabin side, just up behind the glove box, behind a plastic cover held with, I think, one screw. The tube nipples stuck straight through the firewall, where the hoses attached in plain view. Then you just reached behind the glove box and pulled the heater core straight back.

 

Probably a dealer-installable option kit then. That used to be a lot more popular.

I remember a friend buying an early 90s no-air Miata secondhand, and the dealer offered to retrofit the kit to seal the deal.

 

It was just the way they built 'em; shown that way in the regular manual, not as some sort of accessory.

It was a good way to build 'em too.

I had an '88 323, its manual showed both a factory A/C (that I didn't have) and a dealer-installable option (that I also didn't have), but the dealer one was pictured as kind of an obvious wart bolted on under the dash.