Dusty,
I have been thinking about your idea for several days. I have built two Model IVs and try as I might, the firewall seals were not perfect on either one. I put 900 hours on the first one and about a hundred on the second one. I guess I am curious of what exactly you are wanting to correct or prevent by the modification you are suggesting. One of the things I really like about the earlier models is the ready access to the back side of the panel, the pedals and the brake master cylinders by simply removing a couple dozen or so Clecos. If you are concerned about the CO possibility, my suggestion would be to put a monitor on your panel for a glance or two while flying. As I mentioned 900 hours on the first one and much of that was alongside other IV owners and none of us ever felt an issue with that risk given the typical "ventilation" elsewhere around the cockpit.