I have a dead stock '66 Starfire. It has a 10 bolt differential, but GM made more than one design of 10 bolt over the years. I'm new to GM cars (old Ford man) and could use some help. Which 10-bolt do I have???

 

Picture would help A LOT Jim

 

If it's the original rear, it should be a P-type (Pontiac) 8-7/8" 10/12 bolt.
That's 10 bolts on the cover and 12 bolts on the ring gear. Looks like a regular 10 bolt but bigger & heavier.