My Dad bought my car new. It's never been wrecked although it was resprayed on the outside in the '70's with the same color. Maybe you can determine some of the undercoating stop points from these pics when I had the suspension off replacing the bushings. Looks like the factory used some sort of square blocking panel to keep the undercoating spray from entering the engine compartment. (no undercoating seen when looking in from the top)
On the frame rails it seems my undercoating stops about where the rail passes the firewall on the top but were undercoated most of the way to the front on the bottom and outside (as evidenced by the tire rub).
OK - that's 50+ years of road grime so don't laugh......
The proportioning valve for my front disc conversion is not original but you can see the white frame rail exiting out from under the firewall.
Both the front and center cross members are body color on my car. Notice there is no undercoating overspray coming through the suspension opening and only paint on the top suspension mount inside the engine compartment.
Pretty bad pics but I was taking them for other reasons (showing my A/C addition on this one). I'm sure someone will have some better ones.
Hope it helps though.