Found my dads original 1971 duster!

dads71 said
Also, there is no fan shroud on. I bought one, but my rad….-although verified to be the proper original rad doesn’t have mounting holes/brackets to allow it to be installed. Car didn’t sit and idle though. Drove it 5 min up the highway and 5 min back
I’m going to try to figure out how to mount the shroud in some other way, but I’m thinking that shouldn’t be very important unless car is just idling

At this point, I wouldn't be surprised if the t-stat was installed upside down.....I've seen that before. Also, maybe they forgot to install the lower rad hose spring inside the hose and it is collapsing from the draw of the water pump.....2 easy things to check.

EDIT: Could be an air lock in the system as well.