Check what size studs are off the end of the axle tubes that hold the bearing & axle shaft inplace just behind the drum brakes.
Once you take off the drum you can see them.
If you have 1/2" bolts its a large bearing. 3/8" bolts are a small bearing.
There is a stamping in the backing plate at either 3 or 9 o'clock that will give a # and I can tell you for sure this way. But to tell you the truth if you have gone that far you might as well pull the axle shaft and measure the bearing.
2-7/8" - Small bearing
3-1/8" - Large Bearing
Some go by the rear drum but it tough as some years were mixed.
66-75 came with 10x2.5 drums which is small bearing.
66-75 also had 11x1.75 which is usually large bearing but can be mixed.
76-77 should all be large but I've heard some are mixed too.
