p206-207 2007 NHRA rule book, section 16 2:4 "In place of a crossmember, 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 minimum thickness and 2 inches wide, or 7/8-inch x 0.065-inch welded steel tubing, securely moounted and located within 6 inches of the front universal joint for support of the driveshaft in event of U-joint failure." It also has a nice sketch.

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68 Torino GT FB 351W 4spd