If you're going to strip the underside, might as well refinish it like original, unless the guy wants it "hopped up": that means a medium grey color (unless it's an L.A.-built car, they were flattish black), with body color overspray from spraying the lower perimeter of the car, and then undercoating in the wheelhousings. If the car had factory undercoating, that's another option of course. Looks like the rad support is already blacked out.