318 MAX fuel economy builds?

So I'm looking into buying a long-distance capable daily driver. I have my eyes on a Dodge Diplomat and Chrysler 5th Avenue. Both have 318s and I thought it may be of my own interest to buy one as a daily as they are things I can work on instead of things I would need to take to a shop. In the case that I buy one or the other I would like to build the 318 for high fuel economy, I'm talking like 20+ mpg if possible. Anyone done this? Any input? Etc. Let's coffee-table talk daily stuff instead of high horsepower for once.
Guess I'm in really late on this one. My wife had a 1985 Diplomat we bought used. Dead stock 318 (Lean Burn) with about 85,000 miles on it. Never touched the engine except for oil changes. Ran beautifully despite the Lean Burn and got 23-24 MPG (2.24 axle ratio) all day long on the interstate at 65-70 MPH. Beautiful riding and handling car. Not a stoplight sprinter but it was a solid car for the money.