In earlier times, in many vehicles, the instrument panel shell was a part of the structure and painted when the vehicle was painted. That was when there was not much complexity in what was in the instrument panel.
As things evolved, the instrument panel came to not be a part of the car when it was painted, being assembled "off-line" at the assy plant. Then installed as a unit into the vehicle structure, on the main assy line. This allows for much easier installation of the hvac module, pedal related items, and other things attached to the firewall/inner cowl area, with the inst pnl not being there. And, of course, the front seat items are not installed yet, either.
When we did the Superbird restoration in the early 1990s, taking it to Mopar Nats in 1992, we replicated as many OEM-production ways the car was assembled at the assy plant. In the case of the inst pnl, it was put together on a long, empty work bench. After it had been painted, everything was put back into it, on the bench. When completed, most of the interior was installed, except the front seat.
On the night we installed the inst pnl, one guy was on each end, holding it carefully and firmly. Then they "walked into" the front seat area and carefully positioned the completed inst pnl to the A-pillar and inner cowl area, as others had the needed bolts and wrenches to install the bolts to those body areas and across the front of the panel to the upper cowl area. Took maybe 10 minutes? Then the steering column went in next. Plus making sure everything was securely plugged in at the buokhead connector. Then the flexible defroster ducts, radio antenna coaxial cable, and anything else that needed to be plugged in.
On that car, the windshield was not installed, so that made for a much more open and accessible environment to work in. It was also a bench seat column shift car, so no console mounting brackets to trip over, either.
Doing this car in the same assembly orientation as the assy plnt proved the viability of this approach, to us. In most cases, easier and less-involved than trying to do things as we might perceive as "normal" in the field. The engine, front frame/transmission were installed from the bottom, which meant NO issues with chipping the fresh paint on the inner fenders or having to install the hood, afterwards. Having a body-contact lift was necessary, which was in the shop.
Just some thoughts and experiences,
CBODY67