will only 1 wired Temp sending unit work gauge

If memory serves it is one wire. Should be green and the sensor on the drivers head between cyl 1-2.

That's what Rockauto.com is showing for your vehicle with either the V8 350 or 454. To be sure it's the wire for the temperature gauge, touch the wire to something metal that's not painted. The gauge on the dash should immediately swing to hot. Hope that helps! Jim

Do the test with the ignition on. Jim

I have temp sending units on both heads on a 350

Do you have automatic climate control? If so, one of the sending units is for the climate control system. In the wintertime the blower for the heater doesn't come on until the engine coolant warms up to about 100°F. The sending unit on the passenger's side is probably for that. Hope that helps! Jim