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Re: Eight MPG?!
A 350 is not exactly a "small" engine and is certainly powerful enough to power this truck with no problems. I wouldn't go as far as changing the engine to achieve better MPG. Sure, smaller engines work more than larger ones to power the same vehicle, but again, the 350 is ample. 4:10s on 33 inch tires is about right for an efficient performance in the powerband. If you get much smaller tires, you will run higher RPMs. The tread definitely has something to do with it, though, so a good street tire will produce less resistance when compared to an aggressive mud tire.
I wouldn't mess with the more expensive synthetic oil, either. I would perform a complete tuneup including cap, rotor, wires, plugs, etc. and then reassess the situation. Check to make sure the plugs are correct for your engine (hotter vs colder plugs). I believe you should be running R44T or something equivalent. How did you calculate the mileage? As far as I know, these trucks either came with a 16 gallon or a 20 gallon tank, so make sure your calculations are right. An open element air cleaner will typically get you better gas mileage in theory, and the weather in Oregon right now is pretty nice. My '73 K20 came with a snorkel type air cleaner, by the way.
The exhaust seems pretty restrictive, and if the engine is at the very least built a little, you probably need larger pipes. Also, check the CFM on your carb model. Depending on the Edelbrock carb you have, it is likely a Carter AFB copy. These are easy to rebuild so you might even do this if you think it is necessary. You do not need a carb larger than 600 or 650 CFM depending on how modified the engine is. Mild builds are good with this size carb.
The old '77 family Suburban from my youth with a 454/auto got over 9 MPG, so your 8 MPG does sound too low.
M
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