The Firewall was blacked out as they say, before all the front end components and bolt on parts came together.
While the transition point on the fire wall of black varied greatly Plant to Plant,
some stopped at the Body seam and others went straight across the top of the cowl.
That said you will always see a difference in "GLOSS" between the Firewall & Bolt-On parts, i.e. Air Box. Inner Tubs and such.
To generalize it all one color in not really accurate.
The black-out is a low gloss while the other component since painted separate during the assembly process are more or less Semi to gloss.
Underside of the hood also follows the firewall Black-Out " Degree of gloss.
Take some time and search for True Survivors for reference and you will start to notice the varying gloss under the hoods.
Avoid HIGH GLOSS like so many of the over-restored show queens.
GTO Jr. - A.K.A. SPRINT 6