Yeah all the goals are in line and easy except for the fuel mileage part..."solid double digits" are you talking 12mpg or 22mpg?
The keys to that will be
1. Put an egg on the throttle pedal
2. Get an O2 sensor and tune idle and cruise values as lean as you can while maintaining response and idle quality.
3. Run low gears and better yet an OD trans with a lockup converter.
4. Stick with very mild cam timing. I.e. stock.
5. Ideally go with EFI this will help with the lean mixtures
6. Run a properly curved vacuum advance distributor.
I'd be looking for a 5.9 magnum. It doesn't have to be super low mileage. I'd be fine with a sub 150k mile engine, up to 200k. It may need some basics like a head gasket/valve job/valve seals. The bottom end will probably be fine ifnit was horrendously abused. They're easy to find and not expensive. I personally wouldn't bother with the 5.2. Many here will disagree with me on that point. But dollar for dollar the 5.9 will be up on torque literally everywhere in the curve and the only way to close that gap with the 318 will require sacrifices and money.