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what was the orginal rear end size for a 1969 Mustang Mach1 "M" code aka 351w with fmx trans. My dad said it was a 9in bust some guy we met claimes it wasnt. And also what is the gear ration in the rear end? Sorry i am not a rear end and trans guy but i do know a motor.
 

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I pretty sure it would have been the 8", the big block cars got the bigger 9" and the 428 super cobra jets were upgraded to the heavier 31 spline axles. The 351 engines (H and M codes) aren't big blocks but if by chance your car did come with a 9" it would have been the lighter duty 28 spline, same as the 8".


There were several axle ratios available, and you'd have to look at your door tag if it's still there and original to the car or order a Marti report on your car.
 
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