I mounted my radiator vertically and closed in the sides and top with sheet metal. I do have temperature problems in traffic or on the highway. I can get to 220F at the engine but the coolant from the radiator is less than 190F. I have a sender in the return hose. Being that it is vertical, do you think I'm not getting air at the top? I do have a shroud that is 1" offset from the radiator and a 16" fan with variable speed control.
As an experiment, I mounted 2 bilge blowers in front of the radiator at the top to see it it makes a difference. I don't plan on keeping them there because they are noisy. The air exits through the bottom of the trunk. Would tilting it help? I have a high mounted expansion tank and electric water pump. I will test it in 2 days going 50 miles to a show if the weather holds but the forecast calls for high in the mid 60's so it may not be a good test.