Brian,

I use only the well nuts and not the fuel tubing. The well nuts are thick enough to be able to bite into the firewall, but soft enough to allow the engine to float some. The key part is to make your well nut holes in the firewall just the right size. Practice on another piece of lightply with different bit sizes aand mount the nuts. try and pull them, twist them, etc. then take the best bit size and drill the firewall.