65 Valiant now gauges go full when ignition key turned or running

A dedicated ground should attach from inst' panel to chassis ( as in chassis ground ). Factory likely would have routed it to metal behind nearest kick panel. Find ample examples of this practice in your local parts yards.
You have probably read "attach to steering column" in this forum. Steering column is a separate part. So is the dash. Example scenario, you wish to pull the panel out a distance for in car testing. Lowering the steering column for this interrupts the ground you added. The word 'silly' comes to mind again.
Best place on the inst' panel to attach the ground... secure I this vibrating environment means ring terminal and toothed washer just like the factory used at ALT' gauge. The little sheet metal screws in circuit boards are not long enough or into enough material.
Pic of backside of your panel will help me help you.