There's several other threads on this topic, from which I've gathered that you can average about 20-25 with a tuned truck having dual carbs, headers, t-5, appropriately geared rear differential, electronic ignition and PCV valve.
The theory is with the dual carbs you are getting the fuel to the cylinders more efficiently (less waste), that combined with headers, electronic ignition and PCV valve you are maximizing the engine's efficiency. The tranny with overdrive and rear differential with higher gearing (3.73:1, 3:42:1, 3.06:1... in stead of stock 4.11:1) you are able to get operating speeds without having to rev the engine real high and burn more gas. In otherwords, maximizing the efficiency of putting the engine's power into the tires.
Tuning and engine performance are only gonna get you so far in terms of gas savings if you have to keep the engine in the 3k range to maintain 55-75 mph or do lots of stop and start driving.
