as i recall the side gears that the axles fit into are the same between the 8 and 9 inch rears, and the 31 spline axles are only slightly larger than the 28 spline axles. you will need a special axle bearing to fit the larger axles in the smaller ends. but 31 spline axles will fit the 8 inch rear.
I can see them fitting.
The "big" 8" bearing is the "small" 9" bearing.
I am sure someone made small bearing 31 spline axles.
Then you just swap 31 spline collars into the 8" diff.
The collars swap, and 28 spline 8" is 28 spline 9".
Now, why would someone do this?!?!
The 28 spline axles are not known for breaking in an 8" rear.
They are the strongest part in that unit.
There are a dozen other parts in the 8" that will break long before an axle!
Big waste of money!
With the cost of aftermarket 8" parts, the ones that need upgrading, you will spend the same or more than just getting a mild 9" that will be stronger...
The "big" 8" bearing is the "small" 9" bearing.
I am sure someone made small bearing 31 spline axles.
Then you just swap 31 spline collars into the 8" diff.
The collars swap, and 28 spline 8" is 28 spline 9".
Now, why would someone do this?!?!
The 28 spline axles are not known for breaking in an 8" rear.
They are the strongest part in that unit.
There are a dozen other parts in the 8" that will break long before an axle!
Big waste of money!
With the cost of aftermarket 8" parts, the ones that need upgrading, you will spend the same or more than just getting a mild 9" that will be stronger...
uhmm i have never heard of a "big bearing" 8" rear end. to my knowledge all 8" bearings are the same size.
4 lug 8" rears generally use the small 8" bearing.
Don't know about Stangs or Falcons, but this holds true for 4 lug:
Maverick/Comet
Pinto/Bobcat
Stang 2
You cannot swap one of these cars to 5 lug by just putting in 5 lug axles due to the large vs small bearings.