[Longhorn Man]

On a low buck aproach, I'll try to clean the firewall real good... some sort of awsome cleaner, like Spray Nine, or simple green, super clean is another real good one for this. I've even gone as far as getting the red scotch brite pad with the cleaners. Clean it all up, it id looks presentable, then just shoot some clear from a can over it since the pad made it all dull.
If it still looks like ass, or isn't the same color as the truck's body, then I like seeing it in gloss or semi gloss black. Personally, flat just looks too unfinished, or too far removed from correct.
Inner fenders... always gloss black IMO.