Temperature gauge does not work on my 1970 340 Duster ?

And thats why placing the sender wire directly to ground doesn't really check anything but the wire. Zero ohms resistance will get results that within the normal 80-10 range will not. A cooked gauge will be way out of calibration but in that case the needle wouldn't go all the way to home at off either.
My solid state regulator examples are of those housed inside fuel gauges. Those must be a adaptation of universal type regulator. The RTE unit may be a bit pricey but it is a plug and play solution.
Regardless what route you take ADD A CHASSIS GROUND WIRE TO THE INST' PANEL. The factories daisy chain of a ground path ( through parts and mounting hardware ) is unreliable and too easily degraded or broken. The T805 solid state bit used in most of the replacement regulators will pass full 12 volts when not grounded.