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I have a 1968 Convertible that was a base with 210 HP and 2:73:1 gears with a Power Glide.  I am in the process of upgrading  and would like to convert the rear end to a lower gear (3:55 or 3:73) with posi traction. Is there a kit to make this change easily? If so, does anyone have a pointer? If not, any suggestions on making this change? Thanks!