Found my dads original 1971 duster!

Have you pulled your thermostat, make sure its not in backwards? Put it in boiling water make sure it opens?

I would drill at least one 1/8 hole in thermostat to insure no air lock exists. I have three 1/8 inch holes in mine

North of 200 degrees F and would not be comfortable with that on my car. Over heating can cause heads to warp and a host of other issues you don't need. My two cents.