I will try to say this as nicely and politely as possible. I don't want to sound condescending. 17 MPG is good for a 340. The 340 was a performance engine. It was never designed or expected to be a 'good mileage' engine. In fact, the 340 is one of the few 'performance only' engines ever made. You could not get the 340 in a station wagon, truck or a 4 door car. It was a performance option only available in a 2 door car. There have been reports of 340s being put in station wagons, trucks and/or 4 door cars, but they have not been substantiated that I know of. Most other high performance engines are variants of engines available in all other cars and trucks. When Chrysler put the 340 into their cars, it was as a performance platform. It was never meant to be an engine to deliver maximum gas mileage. Personally, I think if you are getting 17 MPG, that is about as good as you can get. You might be able to boost that a little with tuning, but any significant increases might get expensive and destroy the performance you have from your 340 now.
You could get a highway gear put in your car. You could put a 2 barrel carb and manifold on it. You could rebuild the engine with lower compression and fuel mileage in mind to include a milder cam. You could put fuel injection on it (some people on this forum have claimed better gas mileage). You could possibly put some slightly taller tires on the rear. You could swap a /6 into it and store the 340 in the corner of your garage. That way you could always put it back in. In my opinion, the bottom line is that the only way you can significantly increase your gas mileage will be expensive and lessen your performance. People are going to disagree with that statement, but please notice that I said 'significantly increase'. I am not talking about the small increase yo might get from tuning, and it sounds to me like you are looking for a significant increase.