I'm not familiar with the '66 panel so I'll make some assumptions. The guage itself penetrates the backshell of the cluster and is held in place with 4 insulating washers and 2 nuts. Harness wires attaches to the same two posts.
First check is to remove the wire at the guage that leads to the sender. Turn key on. If guage still pegs, the problem is with the guage. See #1 below.
1. The most likely fault is for the guage to have shifted in the cluster and one of the posts is touching (shorted) to the back shell. In this case, it must be the side connected to the wire leading to the sender unit. Remove your cluster, remove the nuts, center the posts and fiber washers and then replace the nuts and tighten enough to make sure the posts don't slip around.
If removing the wire to the guage results in the guage reading zero with the key on then do test #2 below.
2. The next most likely fault is the wire from the sender to the guage is shorted. It is chafed has a bare spot or a sheet metal screw screwed into the harness causing the short. Disconnect the wire at both ends and use an ohmmeter or electrical test light connected between the wire and the car frame. If the ohmmeter reads near zero or the test light illuminates with both ends of the wire free, then you have a shorted wire to fix. OTOH, if the ohmmeter reads infinity or the test light doesn't illuminate, then the wire is likely good.
If you still haven't fixed it, do #3 below:
3. The next most likely fault is the sender. If the sender is shorted at the upper terminal, you'd have the fault you see. Use the ohmmeter to test the sender with the engine off. Connect the ohmmeter between the sender post and the engine block. It should read infinity or a reading higher than 200 ohms. If it reads lower, tell me what the reading is.