Need guidance on getting 340 running again

After siphoning out what you can get out (I recommend this Performance Tool W54154 Shaker Siphon Hose | Google) you can make up a long piece of 5/16 inch fuel hose and add a short piece of steel line to it to drain the rest of the fuel. Pinch the suction hose closed (pinch off pliers) into the fuel pump closed, disconnect it, and connect up your steel splice line to it and run your drain hose downhill to a clean five gallon bucket. Use a drain pan during the change over to catch the spillage, and you can take the discharge side loose later and catch what runs back from the line to the carburetor. You can set up a clean bucket and pour a couple of gallons through the tank to get an idea of what the fuel system is like from there. If it's the original tank and lines with the vapor recovery setup, I wouldn't be too surprised to find that the tank and fuel line both need replacement.