Temp Gauge or Overheating??

[Green Machine 79]

Hi folks,

I have a 1979 C-20 5.7 350.

It sat on edge of my farm field for near 3 years before i finally got around to resurrecting it a few weeks ago. Just needed a new ignition coil and distributor cap to get it running.

BUT, now the temp gauge goes to hot within 60 seconds of starting up.

I switched out thermostat, Topping off coolant. No difference.

Changed out sending unit. (the unit offered both online and at oreillys for my particular make/model has a spade type connection for the wire, tho my original unit has the brass knob. But the connector piece that is on the wire does seems to connect well. Not sure if that could be a problem?? But otherwise the units look identical.)

But no change in Temp Gauge.

I also placed the sending unit in a glass of hot water while connected to wire but couldn't really tell if gauge changed much from C.

So I got into the gauge cluster and am getting the same reading on each side of the wire from sending unit to gauge.

If i remove wire from sending unit, my gauge drops to C. If i ground the wire it shoots up to H again.

The speed at which the gauge reaches high varies. This morning, having not started it in 24 hours, the gauge took about 4 minutes to reach high. That may be an average of what it takes tho often its much more instant than that.

After running it to hot, then stopping it for about 15 min. The gauge read only about 1/4 way up from C before starting. (Went to H within 30 seconds)

I ran it with radiator cap off and am not seeing much if any change in coolant level as it heats up. It's swishing around inside and is fairly hot to the touch by the time the gauge is past H.

Should i turn my attention to the radiator? I figure i should flush it today and while it's empty go ahead and take the thermostat housing apart and maybe boil check the thermostat since not every part I've bought from oreilly's has been functional.

Any ideas would be really appreciated. I'm running out of them. Thanks!!