I wouldn't get too hung up on spinning the engine slower just to keep fuel consumption down. You will need to match the final drive ratio to the engine combination. You will get better mileage running your engine at a reasonably built torque peak than you will be simply turning it slower. If your out of the power band, then you will have to apply more throttle, more throttle angle results in more fuel consumption, regardless of rpm involved.

The 440 in my truck will produce better mileage figures at 3200 cruising rpm , just under its torque peak, than it will at a steady state cruising rpm of 2500. But, my engine combo is built to be optimized at 75 mph, which coincides with that 3200 rpm number. That is achieved using a very small cam. To get anything smaller than most basic off the shelf variaties will require some custom grind of some sort.

I'd also figure some way to use the magnums. They are a newer head which will get better results for flow, swirl, tumble and the resulting burn than the 40 year old 273 heads. A lot more thought and effort went into those magnums than the earlier LA heads.