[Speargun]

I don't think the shroud is the problem. That fan is 19" iirc and has plenty of open space for air to flow through.
The first thing I would do is to make sure that there is a spring in the lower radiator hose to keep it from collapsing at the higher RPM's.
When it comes to cooling, faster flow equals better cooling. Try removing the thermostat and see if the engine still runs hot at speed. This made a HUGE difference on my bird.
Knowing what your oil temps are would help. It's possible that at the higher RPM's, the oil temp is climbing and adding more heat to the engine. I had that problem in my '67 with a 455. Which oil are you running? If the clearances are good, 10w-30 should be all you need. Anything over 12-15 psi at idle & 10 psi per 1000 rpm is fine.
As mentioned above, Timing....
Other things that help are the upper & lower baffles that came in AC cars to help direct airflow to the radiator. I also added some of the black foam, water pipe, insulation that you can find at Lowes/Home Depot between the radiator and radiator support so no air escaped around the radiator.

If you have a spring in the lower hose and removing the thermostat doesn't help enough, you may need to remove the water pump and verify that the divider plate is as close to the pump vanes as possible. I try to make mine no more than around .030". It could be up to a 1/4" away if it was never clearanced.