For the tranny, I'd source an AL OD 833 (light weight and more efficient). For the rear end, I'd source a 7 1/4 rear with 2.94 gears for a 2.2 final drive ratio (lighter rear/gears/axles reduce losses/rolling resistance on several fronts...the closer to 1:1 a gear is, the more efficient it is with regard to frictional losses.)
A four speed with a direct drive high gear is more efficient than a overdrive trans. You can get a 2.2 something ring and pinion out of a M body. You can swap a 3.09 low gear, direct drive high gear set into a aluminum case if you want the weight savings.
