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.

We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon