What Heath said. According to my '84 shop manual, there were three different sending unit assemblies available. The 6.9l has one with two lines; one for fuel pickup and one for fuel return. The 460 sender also has two lines, and an electric fuel pump integrated into the sending unit. The other gas engines have a single line, as pictured.
I think the boards are the same, so I imagine it would be possible to change the board over, but I've never done it. '80-'86 should be the same, but they changed the resistance readings for later trucks, so this would only work for an '80-'86 truck...