ok, i checked out the rear end swap sticky but didn't find an answer to my question. i have a 57 big ford axle housing and i'm putting in a 70s 9 inch 31 spline carrier. i need to buy new axles for this application. i found some on summit but i need to know the bearing and seal size i need.
The rear axle shaft bearings and seals came in 2 sizes. Could have been the standard car or heavy duty like a wagon or Ranchero. You will need to measure to be sure. My '57 rear took the smaller bearings.
If you have an axle out of your 65' Mustang and it is an 8", temporarily swap it into the 57' housing (with no pumpkin.) It you have the small bearing, the bearing will fit perfectly. I've also used 65' Mustang replacement 31 spline axles in the 57' rear using the small bearings. The axles didn't hold up as well as the "good" aftermarket axles as they were Chinese made stock replacements. Weren't up to dragstrip use with a stick shift and slicks, but probably OK for the street.
IIRC, you would still use the 57-59 style small bearing axle seals though with this combo.
As mentioned, large bearings where typically used in the Rancheros and station wagons. Full sized cars normally got the small bearing ends.