What I can tell you from my original-paint cars is that the compartment & hood underside were pretty much covered, but not as well done as the exterior.
In some areas (backside of shock towers, for example) the paint was pretty thin, as were the frame rails below the top horizontal flange.
Backside of underhood contours was pretty thin.
The paint at the firewall had runs in a couple of mine.
Top of battery tray (under battery) tended to be fairly thick (bridging fasteners/tray).
Basically, a guy leaning into the compartment, waving the spray-gun kinda hurried, and a lot less care than we take restoring them...