farm boss

I got a 76 custom deluxe, 4x4 3/4 ton, 4 speed w/ creeper, 400 sbc, The front axle is a dana 44 and rear is a corprate 12 bolt with 4.10 gears. I knew when i bought this truck with that combo that id be looking at the worst gas mileage ever, and i was just looking for ways to increase the mileage, which is crazy. I could care less about raw power and torque at this point lol. I was thinking headers, carb spacer(maybe?), dropping an automatic turbo 350 tranny in it, and changing the gears in the axles. Just looking for some advice to what would work best for this truck