rearend under '71 cougar same width as

[70SWGR]

the rearend under a '69 mach I?
Would both have 31 spline axles?

 

[HHStang]

My guess would be no. The 71's were like 60" hub to hub, the 69's were maybe 57"?

 

[chepsk8]

Cougar & Mustang shared all chassis parts. The rears from 67-73 will fit.

28 spline in all, unless it's a Drag-Pak rear. Another way to get 31 spline is to find a Lincoln or wagon pumpkin, and change the gears, and find the 31-spline axles.

 

[HHStang]

center sections may be the same, pretty sure rears are not (width, etc). See this link.
http://www.mustangsandmore.com/ubb/FordRearDimensions.html

 

[rbohm]

yep 71-73 rears are wider by a bit. and as said 31 spline with drag pak cars and some big blocks.