C20 12 Bolt

[slimmark]

I have a 67 C20 with a 12 bolt rearend (sitting in the garrage - the frame is on the driveway) I am in the front end of a ground up rebuild. I want to replace the current rear with an eaton posi unit - which one?
The tanny is a 4 speed manual with the granny gear - my driving is all Hwy - no stump pulling for this truck.
What is the best way to get better gas mileage - change the pnion and gear in back or change the gears in the tarnny.
While I can pull out and tear down an engine I have no experience working on a transmission. All help or comments are welcome.
Thanks in advance