The 3.7 should be almost as easy as the 3.9, with better results. The 3.7, being based on the 4.7, already has the same bellhousing pattern as the "A" engines, so it would bolt right up to a 727/904 or any other "A" engine backed trans. The deck height and width should not be an issue either.

If you went 4cyl, you'd want to go with the 2.4/2.4 turbo. You could also use a 2.2/2.5 turbo. None of the rest of them are worth swapping. They did make 2.4L 4cyl 2wd libertys, so a trans and oil pan should be able to be found.

If I were you, I'd do some reading on quench, and learn how to install/tune megasquirt. A high compression, good quench, small cammed, fuel injected 318 magnum with an OD trans behind it would get seriously good mileage.

Realistically, the aerodynamics aren't great on A-bodies...so getting over 30mpg at any real speed would be very difficult. I believe the aforementioned 318/OD trans combo would provide good performance (275-300hp) and could get very high 20's for mpg.