First I thought it best to rivet on the recess in the firewall, did that and sealed with high temp sikaflex. I had noticed that the centre rib was riveted in the wrong spot, not sure how this happened, but when I checked the top skin line up with the rivet holes, sure enough it was out. I had to drill these out and carefully placing the top skin back on I marked and drill through the firewall again. I then had a alodined scrap strip which I riveted on where the wrong holes were and proceeded to rivet the rib into the right spot. Only a builder will be able to pick it because now there are two lines of rivets running down the centre firewall. I was very angry with myself on this one.
