the 69/70 and probably 71-73 cars like the Boss cars and the drag pak cars got 31 spline small bearing 9" rears and i believe some early brono's as well. kinda slim pickins anyway you look at it though.
I've got 31 spline late model Mustang sized bearings in my Ranger. These were shipped by mistake and the guy who did the narrowing job was clueless that these were not what I wanted till he was done with the job. The axle ends are small bearing with the Fox/Ranger brake pattern. So yes, you can have 31 spline axles in an 8" rear. I wouldn't recommend it with the trouble I've had with these. There's no room for a thru bolt/nut retention for the bearing, the bolts are fine thread and the threads in the housing are the only thing holding them. Hence they've worked loose several times. The tapered bearing doesn't help either, the retainer plate is constantly under load.