Actually, I didn't shave the inside. I'm sure it looks a lot cleaner, but I figured with dynamat (or varient) on the inner firewall and the a/c hanging down, no one would know but me,
. I filled each individual hole, some guys cut out the entire (or most) of the firewall and use one piece of metal to remove the holes. That's probably how they end up with a smooth firewall on the inside.