[Aussie Mike]

You may not even need to run the duct outside the body. There's a heap of air pressure built up inside the wheel wells. remember the Ferrari 250 GTO had huge cut outs in the rear guards to let the air out.

I reckon all you really need is a small air deflector on the bottom of the diff housing to push air flowing under the car up around the brakes. The XU1 Torrana used a similar idea to cool the front brakes. They had a small metal chin spoiler on the front that deflected air towards the insides of the front wheels.

A small air deflector under the diff would only need to extend down a couple of inches and would leave you plenty of ground clearance. Being directly under the diff it's not going to snag easily because it lifts as the wheels ride over speed humps etc.

Cheers