well, In all reality you should have both in there, steel for the strength and wood for the interior to be tacked to. I put wood around all openings and corners in my coupes so the interior guys have something to go off of, but I also back everything up with a solid steel frame to keep everything where it should be; ie door alingment and so fourth. wood moves about the same as fiberglass where steel does not. Use a hardwood,, as mounting stuff to it will be stronger. do not adhere anything to a exterior panel on a fiberglass body ar have anything touching it as it will "track"through leaving a bump on the oposing panel. So when glueing in your wood try to keep it around door and window jams. I have pics of our inner structures on my web site if you are curious to see some. bestbuyrods.com on the 33-34 page you will see structures