Need help with gas tank ground!!!

[C10StartMstngAtHrt]

Hello all.. Thanks in advance for reading my post! I am a fairly new owner of a 1972 Chevy C10 Super 2WD long-bed with a 350/350 turbo. I'm sure its a fairly common occurance among all of you who have been here before but I honestly have no idea where to ground the gas tank in my truck. I replaced the original sending unit in the behind the seat, factory gas tank but it still continues to read "Full" when disconnected and "Empty" when connected. Is this a grounding problem? And if so, where exactly is the gas tank grounded? And I know it is completely unrelated but is it possible to change the heater core from underneath the hood with a factory air-conditioned truck? Thanks again to anyone that can help shed some light on this for me!

Adam