another thing to consider, I've read the closer the valve is to the cylinder side, the more swirl is induced from that shrouding. on a 273, anything larger than the stock 1.78/1.5 valves require bore notching to avoid a collision between the valve and cylinder bore, so the shrouding effects will probably be about the same regardless of head/valve size. on a 318's 3.91" bore, the magnum's 1.92/1.625" valves may give them an additional boost in induced swirl over a 273 head or '302 head with the small 1.78/1.5 valves...

I'm still not convinced the 273 will give any significant mileage improvement over a similarly built 318. for cost and ease of parts aquisition, I still think a 318 (roller cam block) is the way to go, especially if trying to optimize power and economy.