After alumiprep and alodine the flanged ducts I applied CLKs Silicone Firestop Sealant. It will be the same sealant I use to fill gaps in the firewall. Because the tunnel is at a slight angle, it is important that you assemble the flange ducts parallel to the firewall. They were welded at a 2.4° angle. They are just clecoed in at this point so you can remove them while working in the tunnel.