I recently went through my 12-bolt 3.73 on my 63 and found the gears were mounted on a 4-series carrier. For someone like yourself that wants a higher gear ratio like 3:08, you will most likely be faced with a carrier change. If that is the case, you will have the opportunity to decide on changing over to posi if you don't currently have it. I am not a big locker fan, I have had good results with an Eaton posi unit on several vehicles.
These people were great to deal with- http://www.ring-pinion.com/
They can supply you with any parts you need and are reasonably priced.