I know one thing.
As much as a person would like to "Git er done" I wouldn't do it till the engine was in there, or it could very easily need to be done different after all that.
Yes, you need to look at the "end package" to figure out your degrees of freedom to work with...
I agree to have the engine and radiator installed to see what kind of room he has to work with....
If you use the sharp back wall angle, it may work. But you will loose some cooling capacity of your radiator from the partial restriction to airflow.
But it will be more efficient if you angled/swept the rear face to improve airflow. Then you are making the most efficient use of the whole radiator, and not partially restricting air flow with the "brick wall" for the air to run into. a little slope may help increase airflow and efficiency of your radiator...