Just put a supercharger on it. The blower (centrifugal style) will actually improve your gas mileage by 1 or 2 miles a gallon provided you aren't getting on it all the time. That will easily yield you 300 to the wheels without doing anything internal to the engine.
I don't think there's a supercharger on Earth that'll improve gas mileage simply because they're all belt-driven by the crankshaft and incur a parasitic HP loss. Turbos are driven by exhaust gases and although there's some pumping loss, it's much less than the parasitic drag of a supercharger. Therefore a turbo still won't improve gas mileage over a N/A engine but it'll have less of a negative effect on gas mileage than a supercharger at the same compression ratio and boost level.
The real beauty of a turbo on a 5.0L engine is that you can have the power of a 7.0L engine but still have the gas mileage of a 5.0L engine.