Car is over 40 years old. Anything and every thing could have been changed . Pull it apart and check. . With 3.91 the drive shaft should turn almost 4 times to 1 rev of the axle.
It's a one owner car, and he said he never messed with anything but replacing the slant after it busted a hole in the block. I'll still do that to make sure, but my bet is 2.76 or something taller.
