[mikeinatlanta]

You will gain very little stiffness by bonding the carbon to the fiberglass tunnel. Composites gain stiffness relative to the distance between the face sheets. This requires making the part very thick to achieve the desired results which would normally be accomplished with honeycomb core. Unless you wanted to replace the entire tunnel with a honeycomb sandwich piece, you won't be doing much other than adding weight. To achieve maximim stiffness, you also need to box in the structure. Unless the bottom of the tunnel is closed in, you will have very little structural integrity.
Try this:
Take a paper towel roll and try to twist it like a torsion bar. You will notice that is is very stiff. Cut a slit along its' length and you will see that it loses all of its' stiffness in torsion. This is what you have when the bottom of the tunnel is open.

IMHO, you are better off installing a roll cage or investing in a new chasis.