Fuel gauge only going to 3/4 when grounded?
Float on the sending unit leaking maybe??
Could be that, or the float arm was never correctly adjusted in the first place...
With the tank empty, pull the fill tube and look in the opening with a flashlight. If the float is not on the bottom of the tank (or near it), then it is not dropping fully and could be why you're empty when it still reads 1/2 tank. This would also explain why it never reads over 3/4. In this case, remove the sender and bend the float arm down until it is at/near the bottom of the tank at full drop. Then check your ohms through the full range of travel, lifting the float with a coat hanger or piece of wire.
i pulled the cluster to replace the speedometer a while back, and it was sometime after that the gauge stopped reading.
You may not have gotten the cluster properly grounded when you reinstalled it.
Or just pull the bed and install a new sender.
Only as a last resort. Aftermarket senders are notoriously inaccurate, and you may end up no better off than you already are. Either have the original rebuilt, or you may have some luck finding an NOS sender since it's a '77.
Edit: The D150s have a fairly squared off tank iirc, so you may have some luck with the "linear" aftermarket senders after all (I'm used to A bodies, where they don't work well at all).