MPG Motor Magnum vs. Hemi

If you are set on an engine swap I'd vote for a 5.2 or 5.9 Magnum BUT you need O/D to maximize MPG. O/D will yield more mileage than EFI. A Dodge truck has what you need and Hotwire may be able to help.
Or freshen the 413 with a Gearvendors.