I made a few billet aluminium clamps to hold my fuel lines in place. I used nutserts into the tunnel but I installed them from the underside so they fit flush under the carpet. The billet clamps have a recess in the back to slip over the nutsert. Making them this way was just too much work so I did the rest with rubber lined P clips.
Attaching them before droping the body on was certainly easier. My wife didn't complain to much as I had the body on it's side while we worked.
Use countersunk head cap screws and countersink all the holes from the cabin side. That way they will be flush under the carpet.
Cheers