[MagicRat]

Your mileage does seem to be pretty low.

There are many reasons why your mileage is poor. Generally, they fall into 2 categories: the state of tune of your car; and the equipment it has.

If you are not experienced with auto mechanics, this is an excellent learning opportunity to pick up some service manuals, or do-it-yourself manuals (Haynes, Chiltons, etc) and begin to do some research and do some work yourself. This car is actually quite simple and straightforward to work on.

Go over your car throughly. It's possible the previous owner neglected to do some maitenance, it may need a tune up, including a new air filter, spark plugs, plug wires, distributor cap and rotor, and resetting the ignition timing.

Have a look at the carb and make sure the choke is working properly, that is, the choke blade is notpartly closed when warm, and that the idle mixture (if adjustible) is not set up too rich.

Also, this may sound silly, but make sure your brakes are not dragging. If you can push the car yourself on a level serface, it's probably ok.

Depending on your year of engine, it may have a thermal operated vacuum switch that changes the vacuum source for the distributor from manifold vacuum to ported vacuum as the engine warms up. These are notorious for getting stuck on manifold vacuum, which affects mileage and power. For that matter, make sure the vacuum line is hooked up to the distributor.

As for the equipment that is on the car......because the engine is not original for that vehicle, it becomes difficult to advise you on this. Sometimes high performance parts, particularly camshafts can kill mileage. Some aftermarket carbs, particuarly the Holley carbs can be adjusted incorrectly and use excess fuel.

Finally, it's possible the engine swap was poorly done and something was missed, such as missing or poorly installd vacuum lines, etc.
Sometimes, people can mix-and -match small block Chevy engine parts (blocks and heads) and end up with something that runs and looks ok, but is actually very ineffiecient due to low compression ratios. Do you know anything about the 305 that is in there now, such as the year or what vehicle it came from?

Finally, those old-style GM catalytic converters were inefficient and liked to get plugged up, which kills power and mileage.