what combo = decent mileage+ power/torque

The smaller the displacement, the less fuel it will use and the less torque it will produce all else being equal.

Use cast iron heads, as they will not dissipate as much heat which makes them more efficient. Use closed chamber heads and set your quench to about .040. This will allow higher compression ratios on crappy gas and compression is free power. In other words, use magnum heads. Don't increase
your valve size as you want to keep intake velocity high. Porting won't hurt if it's done by someone who is knowledegeable and may help increase velocity, but just hogging them out won't help either. Good port work cost money.

Your cam duration should be 218 degrees or less @ .050 lift for a good steady idle with high vacuum. A hydraulic roller would be more efficient than a solid or hydraulic flat tappet, as well as broaden your power band, but they are EXPENSIVE.

A small runner dual plane like an old LD4B would work well up to about 5000 rpm, but I don't think I would go as small as a performer unless MPG is your only goal.

Fuel injection would be great for mileage, but your going to spend some money again. Next on the list would be a Holley street avenger.

You might be best off getting a 5.9 magnum out of the boneyard complete with fuel injection and rebuilding it with a hotter cam and MP computer.