I'm thinking you still have an air flow issue. Do you have a winch in front of the grill?

What type of under body armor do you have? You can enclose enough to change airflow characteristics at high speed.

You might try a test drive with the hood off.

What about type of gas?

Do you have ac?

The electric fan should provide a consistent air draw through the radiator without regard to speed. With an intact shroud, you should be good to go. Replacing mechanical with electric is common to address heating issues.

You didn't say which year of engine.

Another thing...

What about your exhaust system? There is a wrap you can put around the manifold to reduce radiated heat. I have used that wrap on exhaust manifold of a 454 in a motor home to solve heat issues.

Also, if your exhaust manifold has any obstructions you will not get hot gases out effectively.