Overheating?

[short72]

Some background...(Forgive me, I am not very mechanical)

1972 C10 402 W/AC
Truck had not been on driven since 1996
Put in a "new" 402 that had about 60,000 miles burning propane.
New gaskets, seals, wires, plugs, etc.

Engine seems to be running hot. Put in a new 195 thermostat and new temp gauge. The old gauge was not working.
Thermo IR gun was showing 210-215 degrees. Did not have the new gauge in at the time of the thermo gun reading.

Now the gauge is showing nearly pegged out at hot after about 10 minutes of idleing.

Took a meter to the gauge while it was running.
Right post read 15 volts throughout.
Left post started at 10-12 (engine cold)
Left post ended at about 7-8 volts (gauge at red)

Any advice would be great.
I will try to give more info if needed.

Thanks a lot.

Eric