I agree with David; the stopper should have a small lip on it and it fits into the tank all the way to the lip. There's a backplate that will be inside the tank and the cap that goes outside the tank. Both have holes for the brass tubing to pass through, plus a center hole for the screw that secures the plug.

As far as the hole in the firewall goes, I'd plug it with a thick piece of balsa and epoxy over it, which will fill the hole in and fuelproof it. Then drill a couple of small holes the size of your fuel tubing in the balsa plug; smear a bit of silicone around them to seal the holes against glow fuel.