Found this online.
In the vicinity of the front universal joint, all cars in competition using open driveshafts, must have a retainer loop 360-degrees of enclosure, 1/4-inch (6.35 mm) minimum thickness and 2-inches (5.1 cm) wide, or 7/8-inch (22.2 mm) x .065-inch (1.65 mm) welded steel tubing, securely mounted and located within "6-inches" (15.2 cm) of the front universal joint for support of the driveshaft in event of U-joint failure. Recommended the loop be round rather than oblong to minimize loading on loop.