If the tunnel is completely enclosed, or the open area is very well braced, it can add quite a bit of torsional and beam stiffness. However, the problem with adding a backbone to a wide chassis is integrating it into the whole structure. It must be connected at the front and rear to something that will transfer loads from the front and rear suspension, or at least the main chassis rails close to them. It can be done, but you may have to add more than just the tunnel.
The "nice" thing about the original 289 chassis was it was so narrow that it was practically a cakewalk to put a central spine on when they did the Daytona coupe. Of course, the 289 chassis
really, really needed the extra stiffness.
One of the most radical of the backbone chassis designs - the Lotus Elan:
