So when you add fuel to the front tank will it run? Is there any chance that the switch on the dash has been worked on, rewired? You can reach up on the front fuel tank and lift up the plug and check the voltage. Did you hear the fuel pump in the front tank come on, with the key to on? You may not want to use the fuel gauge, mine gives me false reading all the time. We can figure this out.
Some people have luck with removing the fuel selector valve and opening the bottom to see if there are any loose parts, open the top and clean it out and reinstall, may work.
1987 F150 4.9 EFI
Mike