When full, my 87 registers 3/4 and my 74 shows 1/2.
Thanks, Jay.On an early car, like your '74, the float is attached to the rheostat with a wire (like coat hanger wire). So you can bend the wire to adjust it. I'm not sure if later cars, like your '87 use the same design.
Also, the sender may not be adequately grounded, since it just grounds through the tank. It may help to add a ground wire between one of the sender attachment screws and the chassis.
It is also possible that the fine wires in your rheostat have worn through, or that the float has developed a leak. In other words, your senders may just be bad.
You can investigate it further by removing the sender from the tank, extending the leads so you can work the float up and down, and watch the how the gauge needle moves.