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I think the 360 ports are just too large for cruise economy.
The smaller 318 heads will velocity up at the low rpm. That's going to give you much better cylinder fill at low throttle opening.

The Magnum heads are simply a better design but may be a tad large for maximum economy.

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The Magnums are just fine for the 5.2/5.9 motors.
The key is to KEEP the velocity up, and a good
design intake and cam WILL do the job. Careful
selection of duration/overlap on the cam and a
dual plane or torque-type intake is neccessary.