Hello, printed information on the dual tank switch over system is pretty sparse and hard to find. GM does not even show anything on it in the service manual. I am going to post what info I was able to come up with below, for you. Note that the fuel lines are supposed to be color coded. It is very possible that they will not show any color after all this time, but the ports on the switch over valve, are numbered and lettered in the picture to follow. That may help. Other wise, you will have to follow the lines to see which one, goes to which tank and work from there. Just use either a fitting to join the lines you need to the main supply line that goes to the motor, or use a new line to bypass the existing set up from the factory. Then just plug off and cap the ones you are not going to use and leave the dash switch turned to the side tank that you are using. Should not be that hard to do, just by looking at which fuel lines go where. Hope this helps. The worded instructions below, are for replacing the valve, the picture will be at the bottom of this post. Thanks
WARNING: To reduce the risk of personal injury when disconnecting the fuel hoses and lines, cover the fuel hoses and lines with a shop cloth to collect the fuel, and then place the cloth into an approved container. REMOVAL: The auxiliary fuel tank control valve is located under the right hand side, center of vehicle, attached to the inner side of the frame.