I also agree with the good suggestions listed above. Something must be wrong with your truck tho. I have a 292 in a '64 C-30 1-Ton that I ran for 9 years. Fully loaded at 10,000 plus pounds, that truck would get about 14 or 15 MPG. It was pure stock, except for a slightly shaved head (done at valve job time), a less restrictive air cleaner and most importantly a Straight-Thru Muffler. These engines like to breathe and the muffler alone will make a big difference. I ran the shorter smaller straight thru GMC V-6 muffler that are hard to find now, but you could just use a glass pack. I'm also a big fan of the original 1-Barrel carb, but some carbs just aren't as good as others and you may want to try to change it. Also, I'm not saying that you don't, but how it is driven and at what points to shift at also play an important part. I'm a believer of keeping the crankshaft spinning (no lugging) and at the same time keeping a light foot on the gas.
Bill
