[Peter Serio]

I would advise against cutting that green wire. As shown in Ken's photo above, the green wire for the temperature light/gauge signal on a '68 GTO is actually routed as 2 green wires joined together at the brass tab that plug into the back of the ignition switch. (The '68 Lemans-GTO ignition switch is a one year only switch.) The 2 wires are joined together but act as a single run (GTO-with gauges) current runs from the back of the temperature gauge, thru the printed circuit, into the green wire, out thru the firewall bulkhead to the coolant temperature sender.

Simply remove that brass tang from the plastic plug and either wrap it over with electrical tape or put a black plastic end on it. Fold the wires back over the main and secure it using a ti-wrap.

In 69 and 70 (if you add factory rally gauges to a car that came with warning lights), the green wire is "twinned" at the 12 slot plug-in right behind the gauge cluster. And the green wire at the ignition switch plug-in is just a single wire. (As a branch from a tree.) Exact same deal as '68 though you need to remove that green wire from the ignition switch connector or else your temperature gauge will have a very short life.