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

As stolen from a quick Google search.

"At one atmosphere, the boiling point for water is 212°F or 100°C. A 10 psi pressure cooker will bring the boiling point to 240°F, but a 15 psi pressure cooker will top at250°F. In other words, the boiling point increases at a decreasing rate against increasing pressure."