If you can still find them, the 1957 Ford 9” rear end fits the 64-66 Mustang. The width is the same and thebperches are in the same location. Also, the late 70’s Lincoln Versailles 9” rear ends have the same perch width but overall width is 1/4” wider. You’ll need to change either driveshaft yoke or pinion yoke to with in your Mustang. Also, most Versailles rearends has 2 something gear ratios and aren’t great for a performance car. You’ll want to upgrade.
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