While commuting 120 miles daily several years ago, I experimented with various power valve settings to see it made any difference to my fuel economy. Officially, Impco explains the fuel mixture provided by their mixer as being factory-set by the shape of the gas valve. The power valve only affects the fuel throttle fuel mixture.
However, I knew that restrictions in any fluid system (both gaseous and liquid) affect the pressure losses at all flow rates. Because the restriction at the gas outlet in the mixer is variable due to the movement of the gas valve, the gas valve's flow restriction is obviously highest at cruising flows and lowest at full throttle.
The power valve simply puts a fixed restriction in the mixer upstream of the gas valve. At low flows, its effect is small (compared with the gas valve's restriction) while at high flows its effect is large and the effect varies with the square of flow velocity. While not having a significant effect on the fuel mixture while cruising at highway speeds, it is an effect none-the-less.
I found that turning the power screw inwards to lean the fuel mixture did make a small improvement in my fuel economy. It also caused a small seat-of-the-pants drop in full throttle power. This fuel economy improvement suggested to me that the standard gas valve (AV1-16) is richer than necessary for my engine.
The only off-the-shelf option for leaning-out the fuel mixture on Model 425 mixer at this time is the lean gas valve (AV1-1644). Even though Impco recommends the lean gas valve for engines larger than 370 CID. I replaced the standard gas valve with the silicone diaphragm lean gas valve on my Chev 350. Upon my first drive with the new gas valve, I didn't notice any effect on normal drivability. However, I regularly monitor my fuel consumption and I saw an immediate improvement in fuel economy of over 10%.
With this valve's leaner fuel mixtures, it would be useful to readjust your power valve and vacuum advance.