I was thinking more about this. You said that your replace the manual selector valve. Do you still have the old unit? Can you install the old one? Since the aux tank works fine and the issue is with your main tank, but you are getting fuel to the selector valve, I'm thinking that perhaps the new valve is defective in some way. Can you remove it and physically check to see if the route is completely open? I would just find the outlet port and blow through it to verify flow. Turn valve handle and blow through again to verify flow through the other route also. I don't know what this valve looks like, but by definition, it is a 3 way valve. Is it a uniform 3 way valve like a 'T' shape? If so, is it possible that your hoses were configured incorrectly?

 

OK, so I misread this part. With these hoses swapped and the truck not running, were you able to switch the valve and have the aux tank keep the truck running? If so, this points back to your suction line/sending unit at your main tank. Since your tanks and sending units are new, did you put on the sending units? Make sure the sending unit fuel connection was same size as the old unit. A smaller size will impact fuel delivery. You may have to go back and inspect the main tank sending unit to make sure there are no obvious pinholes that would allow air into the mix, or possible replace the suction hose that goes from the tank to the selector valve. It may be that the fuel hose is worn enough that it is allowing air into the fuel line.