How do you select the right power valve?

[19Fordy]

I would like to know how to select the correct single stage AND
two-stage power valve for my Holley 390 4 bbl. carburetor with #51 jets.
My goal is to improve the gas milage as my present carb "gobbles" gas. The engine is a stock 239 cu.in 8BA. I live at sea level elevation. I have been thinking about installing #50 jets also, but I don't want to burn the valves so I began to think about changing the power valve. The engine idles at 17 in.of mercury. Your knowledge and experience is appreciated.