Brian is this a Rochester carburetor? There are some things unique to that carburetor that will cause it to run too rich or too lean. The too lean part often centers around vacuum leaks...of any kind (and there can be a few). For the Rochester or any other carburetor make, it also can come from float problems...often one that's incorrectly adjusted so as to shut off fuel flow too soon. And it can be a failed or failing fuel pump. You mentioned that and it could be your problem.
Incorrectly adjusted idle mixture valve can cause it to run too lean or too rich at idle but this will not be a problem while driving. If your problem is only worse while idling or when you let off the gas at a stop sign, this could be a clue.
And also ignition timing could be a problem, but yours sounds to me more like the carburetor either isn't getting enough fuel, isn't regulating it correctly or is getting too much air due to a vacuum leak. If the problem is a vacuum leak, it probably isn't going to be a tiny leak, but I wouldn't rule it out. My vote is you aren't getting enough fuel. If you're not married to the idea of having a mechanical fuel pump, I've had great luck with a cheap electric pump since 1997 and I'd be happy to tell you more.
