The finish can often be pretty shinny (even the straight epoxy primer sealer is smooth and has a gloss)
Since the firewall rearward was shot from below with jets mounted below the "track" jets can plug (not very often) and pockets or vertical surfaces can sometimes be a little thinner the effort was to coat every surface to eliminate the rust issues. You can often find bare uncoated metal right at the back end where the valance would cover later. basically the rear cross member and the pocket formed above it. This is likely due to wanting the jets to shut off before they reached the end of the car. If it went further they ran the risk of the paint/primer raining down on that car or the next one I believe.
Samples of batch colors from 69 Dearborn
Have more but these were handy
It comes down IMHO (since you have your original color still there) to if you want your undercarriage to match what was original there or if a generic batch color/tint will do. Your choice
Chris - your outer aprons were interesting from the point that the application up into the front inner fenders was different from almost all of the other Dearborn cars that year. Must have been a short period during production that they tried that. Though they returned to that in 70 production there.