To correctly do the firewall it should be painted the same color as the exterior. If you have a GN it should be a gloss black, not really a shiny black as they didn't use a clearcoat or buff and polish it like they did on the outside. I have used a paint prep type degreaser like 3-M prep-sol(sorry I dont have a part #) to make the plastic wheelhouse look brand new. After all the dirt is removed wipe them down with armour-all. The Eastwood under hood black could be used on the metal portion of the inner fenders and on the radiator support. Its hard to match the factory finish on these pieces as they were dipped and the coating was applied by electrolisious(sic) leaving a thin film of black on them.
Eric Fisher