Cant cool my 5.9 magnum. Radiator size or add one thing else?

Mine has been running 205 steady since 2004, I'm guessing 80,000 plus miles.
The thermostat sets the minimum running temperature. If your rad was able to cool the fluid to bellow the minimum, or if airflow through it could pull the temp down below it; then the thermostat would begin to close, to maintain the minimum.
But you are running a gutted stat, and the temp cannot go below 190/200. This indicates that your cooling system, as currently set-up cannot cool below that same 190/200. This is dangerous territory cuz that cooling system has ZERO reserve capacity.Any little problem, and the temp will begin to rise.If you miss it, your engine could be toast.
I would not trust my engine to a system like that. I would not take an oh-well attitude.