'69 C20 Change Trans vs Rear End, or Both

[chevyremod]

I've just bought a 1969 C20 camper special that is stock with a 4 speed trans with granny gear. Also 350, 4b carb (edelbrock, had to replace the original), long tube headers, stock ac, pwr steering and brakes.
I'm looking to paint it a color that I want, put some wheels on it i love (5-spoke cragers preferably), change out the interior to a split bench seat, and use it for a regular if not daily driver.
I'd love to improve fuel economy, especially on the highway. I have no way to measure rpms right now but it seems pretty wound up at 70 mph.
I'm looking for an economical way to better my situation with mileage and drive operation. I'd love to be automatic but I'm ok with keeping the 4 spd forever.
Can you give me some suggestions for changes? Manual vs Auto, rear axle change etc.?