Quoting from the article:
"The desired (for this upgrade) 4.10 ratio is only found in the HO52 axles used 1968-1972 (3/4-ton trucks with the 350/auto trans combo)
To go faster, remove your old HO52 or HO72 center carrier (complete) and replace it with a complete center carrier from a 1968-1972 HO52 axle from a Chevrolet C-20 with a 350/auto trans."
According to this, I need an HO 52, not HO72....
