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If you have a '63-'72 A-Body, you'll need new upper control arms from a '73-'76 A-Body, which uses the bigger K772 balljoint. Aftermarket tubular a-arms also use this bigger balljoint. Upon inspection, the first thing you'll notice is the height difference. The B spindle is about 3/8-inch taller than the A, and has a more shapely profile, making it 3 poundss lighter, helping unsprung weight. One other item of interest is the SAI, or Steering Axis Inclination. This angle is the major reason for self-centering steering on cars, more so than caster, and within limits, more is better. The A-Body spindle has a cast-in SAI of 7.5 degrees, while the B-spindle has an SAI of 8 degrees, further aiding in stability.