I stumbled on this comparison from a SC Chevy dealer regarding the three different weight classes of pickups. This is for the new Silverados, but I think our old squares are in the same range. I noticed that modern technology has not improved gas mileage on the heavier trucks very much. The best mpg in my 83 C20 454 was 11 city/13 highway. Like the write up says, you pay a significant gas mileage penalty if you want to haul or tow a lot of weight.
http://www.herndonchevy.com/Silverado-Comparisons.html