67 Dart cooling issues

I think you are going to get forced into a mechanical fan and a shroud.

Please, who made the radiator?

This thing doesn't push water out into the overflow does it?
It has already been brought up, but I didn't see an answer.

The problem seems to be airflow since it does it even worse at hiway speeds.

I have a two row core in mine and even when it gets up to 210 at a light, a block later it's back down to 190

Sure acts like an airflow problem.
I remember someone saying they had a radiator that had been over pressurized and it swelled the tubes.
He had the exact same symtoms.
(could yours have been used before with an engine with a blown headgasket?) and maybe returned?

I gotta ask, The fan blows toward the engine right?