Horsepower with mileage options

Okay guys, here is what I'm wondering. My son and I are bench building options for his bone stock 1971 Plymouth valiant. Current set up is 318 2bbl 904 stock. He loves the car, but has been wanting to give it some pep but wants to retain drivablility and gain some gas mileage. So he is thinking aluminum heads, aluminum 2bbl intake manifold (yes, I said 2bbl aluminum) he says they exist. Dual exhaust and mild cam. Gear change out back. I think that the way to go would be keep the iron heads, stock 340 cam, 4bbl intake either stock steel or aluminum, and carb to match, dual exhaust and a set if 355's out back. Put all the stock parts on the shelf to put back stock if ever wanted. What do you guys think?