Rear-End and gear questions

[Ole Greenie]

Hey Ya'll...Looking to get a few answers concerning rear gear components.
Situation: 1970 CST S/S with an 12 bolt open 3.08 rear gear set/diff out of a 75 C10. I am looking to change the gear set while installing a posi unit.

While looking around for gear sets and posi units, I have come across some confucing info so I am unsure what to get.

1) I plan to install the 3.73 set with a posi unit but if it is a 12 bolt, but does this mean the number of bolts on the rear diff cover or the number of bolts holding the ring gear in place?

2) Is the ring gear an 8.5" or an 8.6"? Are they interchangeable?

3) Is the pinion gear a 30 spline or 28 spline gear? Which one will I need?

Just seems like GM could have made it all simpler by making a large case and a small case housing and standardized internals except for posi and gear sets. Then you have to take into account a housing for either leaf or coil springs.

Any clarification would be much appreciated. Thanks guys.