My '50 with a 216 in it and 4.11 gears got 19mpg, even with two engines in the back driving on the freeway at 55 mph, making GVW about 4600. It really sounds like you need to set your carb and check your vacuum advance, timing, and point gap too. I found that the dwell angle and timing setting made a huge differance in how well my truck ran. If your vacuum or mechanical advance is broken you will get terrible gas milage. Ditto on the power valve. Ditto on the timing being wrong. I like to run a slightly wider than spec point gap for a hotter spark. It just requires re-setting a bit more often. I have a dual point distributor that the 261 is going to get. Also check for fuel leakage, that will really fool you!
