I got a good one

I would start by eliminating the pump AND sender to start with. Get into the kick panel and find the multi wire connector for the rear harness. With a diagram, ID the sender wire, disconnect the connector, and ground the sender. Turning the key to run should result in the gauge quickly going to full. If so, reconnect, move to the rear and disconnect the sender wire. Ground the wire and repeat the test. or do this part first if you wish

Now use whatever you have, big alligator/ battery clamp, or hose clamp, to "get on" say, the fuel tube out of the sender and ground that FOR CERTAIN. Retest with key on. If it works normally, power up fuel pump and see if it is affected.

You may have some odd backfeed going on, but I still suspect a sender grounding problem. The sender is not necessarily "properly" grounding to the tank. This is a design flaw of these senders. THEY SHOULD have a dedicated grounding post/ tab on the sender body.

The original cars had a "jumper clip" over the top of the rubber connector, which snapped onto the sender tube and onto the feed tube. But THAT was bad enough.........what guarantees the feed tube is grounded? it gets rusty/ corroded, etc as it is run along under the body.