The 271 HP HiPo Mustangs and Fairlanes had the 9 inch
The 3 old Ford parts catalogs PLUS the Hollanders I have ... all say no original factory Ford gears lower than a 3.80 for a 8 inch. I myself have pulled close to 75 to 100 8 inch Ford rears ... and never ever saw a 3.80
When dealing with the 8 inch Ford
3.40 and 3.55 are somewhat easy to find.
3.50 is available every now and then ...
Posi rears are very scare ...
Most are 2.79