Ok, the sender is easy. You already found where it goes. That's the place where the mechanical gauge is installed now. In other words, whoever installed the mechanical gauge removed the sender, so you need to get a new one.
For the wire, find the firewall connector. If GMC is like Chevy, it will be the one that has wires going to the starter solenoid. If nececessary, unwrap the connector end of the harness so you can see the color of the wires. You already know you are looking for the green wire, so follow it until you reach its end. On the cabin side look for the same terminal that the green wire is connected to on the engine side and connect your cluster wire to that one. That's all you need to do. As we rocket scientists like to say, "It's not rocket science."
Here are some Chevy wiring diagrams which might be of help.
http://www.64moneypit.pages.qpg.com/...ucks/id12.html
If anyone knows where GMC diagrams are, please be kind enough to post a link.
Ray