Check very carefully. That's a very busy point in many builds. If, (and it's a big if), the main loom runs along the right hand upper chassis rail through this area. It branches off to come up through the tunnel cover to feed the dash, heater, wipers etc. You may also have an engine sub loom coming in along the left hand upper chassis rail and connecting across to the main loom somewhere under the scuttle. It's not uncommon to see any "excess" loom bunched up and "lost" in this area.
Additionally, you have those that like to run fuel and brake lines in the tunnel that sometimes run them along side the upper left chassis rail.
Make sure you either have eyes on the back of the panel you intend to drill through, or you get a hand down the tunnel, James Herriot style! and physically feel that the back of the panel is clear.
That said, it's the perfect spot for it other wise. Just watch if using self tappers, that nothing can move round in the tunnel and chaff on them.
If you are obstructed by wires etc. Consider making a mounting plate to fit on top of the tunnel, that is wide enough to be secured directly into the left and right upper chassis rails.