Can you improve my MPG's?

[Kfort]

I'm a new guy to this site and not much of a motorhead so I will greatly appreciate any sound advice. I have a 1972 C20 Custom Camper long bed equipped with a 350, 3 on the tree and an Edelbrock 600cfm carb (saved original carb). I would really love to slide in a camper and see some country.

The Issue: Primarily, my gas mileage stinks. Secondarily, I don't have a Tach but my RPM's @ 65mph are up. I wish I had an overdrive. I like my current gear ratio between 1st, 2nd and 3rd...if I could only shift on more time and cruise @ 70.

The fix: My truck is all original so I'm afraid to start changing parts so whatever I do, it should be done right and not decrease value of truck.
1. Is a 4-spd manual tranny part of the solution and will the rear diff gears then require swapping?
2. Do I have too much carb and if so, what should I use instead?
3. What size tires are optimum for improved mpg's?
4. Am I missing another component for mpg improvement besides "slowing down?"
5. After making all the appropriate modifiations, what are the most highway mpg's that I can expect?