[Blas]

Originally, the early SPF cars did not comre with an aluminum firewall, just black fiberglass. The aluminum skin was later added as standard equipment.

Rearding the SS heat shields in the lower areas under the hood, I believe the SS material was used because it does not transfer heat a readily as aluminum does, so it makes a better heatshield material down near where the headers are. I don't think there is as much direct heat in the area of the firewall so aluminum was used there. I'm sure the aluminum also supports the throttle linkage better than the fiberglass alone did.
I have seen a few SPF's that have had the aluminum firewall polished-up quite nicely. Quite labor intensive to do with all the items mounted on it. And while the stainless firewall would look more mirror-like than the aluminum is, it might appear a bit "Blingy" to some....

Don't use anything to polish the aluminum that has an oily residue because the oily surface will pick up and hold road dust like a magnet.