The distance between the outer rear frame rails behind the cab increased from 33 1/2" to 37 1/2" in 1973, except the following:

1973/77 F250 4WD High Boys (Dana 60) / 1973/79 F350's on the 140" wheelbase (Dana 70).

Sterling 10.25" introduced in 1985 F250 H/D's and F350's. 1983/89 F250 L/D's have the Ford 8.8" integral rear axle.

Peeps that install 9's in their 1948/56 trucks use this rear axle made prior to 1973, cuz it's the correct width.