Are you sure the previous setup was aftermarket? For a long time it's been a popular mod to swap mid '70s "A" body disk brakes onto '62-'65 "B" bodies. Conversion included "A" body spindles. I don't know if the bearing diameters were different but you might measure the difference between what you took off and what you're trying to install. If its not going on far enough to engage the nut then the bearings aren't seating and you'll destroy the rotors and or the spindles pretty quickly.