whats the best engine for both performance and mileage??

5.2 Magnum with flat-top pistons, ported EQ Magnum heads, mild hyd. roller cam, headers, Air-Gap, 570 Street Avenger carb or maybe ThermoQuad with a spreadbore intake, or stick with EFI...

Hooked up to a 5-speed OD manual with 3.55 gears out back in a Duster or Demon (better aero). I'd say an easy 375 HP (crank) and at least 25 MPG on the highway.