Heres the bracket you need, no sence in drilling holes. I see jegs has one now for 1.00 less then holleys.

http://www.jegs.com/i/Holley/510/20-7/10002/-1

75 Duster, 451 10.87 @ 123.58 NA 97 Z28 6sp., 12.01 @ 115 on a 100 shot 71 Swinger. 360 magnum. 12.58 @ 105 78 cutlass, 469 BBC. 12.70 @ 108 on street tires. http://www.youtube.com/watch?v=Z2kqNmMfheU