need help locating fuel gauge wire 77

[4x4k30]

ok I have have put in a instrument cluster in my 77 gmc crewcab with factory dual tanks from a 79 it has the fuel gauge in the far left bottom corner it was taken from a 79 gmc I can find the wire the wire that goes to the fusebox on the 79 it has the flat fuses my 77 has the old glass round fuses I can not find a wire to go the fuel gauge I did a ls swap and been just running been on one tank with no fuel gauge . I put in a new Pollack 6 port switching valve and was able only to get the blue wire and lite green wire to work it just switches from one to tank to the other but no fuel gauge I have looked everywhere for a wire diagram on 77 but no luck any help would be appericated the dash switch in the 77 has 5 wires to it and the new dash switch only has 4 here is a fusebox pic that looks just like my 77 does