Smallest rear axle in a b body was 8.25 to the best of my knowledge. The 7.25 was a very light duty piece that would last forever behind a slant 6, but it doesn't live as long with a V8, even a bone stock 273 or 318 has enough torque to wear it out. Lots of a bodies had 8.25 differentials, but I am pretty sure they were all big bolt pattern, which of course presents a new problem in that your 68 is likely small bolt pattern axles.
