Fuel gauge sending unit issuse

[1970Custom]

I've been trying to get my '71 up and driving w/o issues (currently drives just has bugs). Anyway the gauge usually reads empty, not key off empty but on the last hash mark. There is a intermitant occasion where it will actually read but only half tank (p.o said it's been that way since he had it). I'm thinking this is a grounding issue with the sender but could it be something else? It's the underbed aux tank BTW (in cab has been removed).

[fordman]

this should apply to the aux tank as well. http://www.fordification.com/fuel-sending-units.htm

[1970Custom]

Do they both have the same resistance readings?