Dino and John are correct. First I'd change the fuel filter in the carb. It's only $5, so it can't hurt. That way you know it's new. The accelerator pump could be bad as well. Do the filter first.
Now, about the fuel tank problem. You fill the left, and a week later the right is half full?
It sounds like the tank switching valve is bad, or that someone plumbed the fuel return line from the fuel pump only to the right tank.
Then again, the tank switching valve should also switch the return line from the fuel pump, to the tank you are running on. Change that fuel filter, then crawl under there, and start tracing fuel lines!!
Oh, and on a 74, there is no frame mounted fuel filter unless a previous owner installed one there.
Also, if the switching valve is bad, a new one will cost you anywhere from $50 - $100.
[Edited by ChucksChevyPages on 08-18-2000 at 12:58 PM]