'66 with a 2.73 in the rear?

[C10Slim]

Is it possible for a 66 to have a 2.73 gear in the rear? My 700r4 is setup for a 3.73 gear because that's what I want to run and that's what I thought was in there. Any whoo, after finally getting it on the road the speedo is way off. So I jack up the rear and do the ol' turn the shaft and count the tire rotations and comes out to about 2.75 turns. Is that possible? Also seems to be a weak or open diff? You can spin one wheel and both will spin but you can stop one wheel and the other will keep going. Is that right?