Something different Building for MPG.

My /6 build:
Stock slant 6. The 7.5 compression is begging for a turbo. An appropriately sized turbo, blow through on a small 2 bbl Holley carb. A well designed exhaust with minimal restriction, built for velocity, with a straight through muffler design. I would run some version of a 4 bbl intake, and port match to the head. The head would get a basic port job, premium valve job, and either a port matched intake, OR a shorty, basic tube header, for maximum flow velocity to retain the heat. Synthetic oil throughout, and (GASP) 4 wheel drum brakes (no drag) with sythetic grease.
Rear gears would be matched to tire diameter, and matched to overall goal. Highway MPG would get tall gears at expense of around town drive-ablity, and around town MPG would get a shorter gear, for better torque production.