[VetteVet]

First things first.
That yellow wire is connected to the key switch and is the old ignition wire for the points. It is shown as the pink/black wire in the diagram. It goes through the firewall block by the master cylinder into the cab and to the ignition switch.
You must have a different wire to power the HEI distributor.

You have to check the yellow wire for power with the key on and it if is hot then you have to tape it off or insulate it somehow or it may short against the block or starter and blow a fuse or start a fire in the harness.

You need to locate the green wire with the slide on clip and connect it to the sender. There are two senders for our trucks, one for the idiot light in the dash and one for the temperature gauge in the gauge dash. What do you have in your dash? The sender will work in either head because the heads are interchangeable. They will fit on either side of the block but they will be reversed.
The sender just grounds the wire as the engine heats up but at a different rate for each sender. If you find the dark green wire just hook it up and see if the gauge or light works. If it doesn't work or it works but not correctly then start a new thread and we'll help you with it.