Horsepower with mileage options

If you want power and mileage, 318s need all the help they can get.
Start looking at making your engine run efficient throughout the rpm-range and make good torque at low rpms. Power comes naturally then.

A 2-bbl intake... come on, really? Thinking about aluminium heads and then choke it right down again with a wimpy manifold doesn't sound like a plan.
Aluminium heads mainly help at higher rpms.
Mileage comes from making good torque at lower rpms, where you do the most driving (<3000 rpm).

Costwise I think you're still miles ahead even with 'extensive tunnel mods' and a A500/A518 transmission. Consider the costs of beefing up a small GM transmission, getting a proper adapter and flexplate to match it all up, linkage adapter.