To expand a bit on Roger's comments, the fan shroud needs to cover the entire back of the radiator. This ensures the full radiator area gets air drawn through it. When the radiator is tilted the fins tend to block natural air flow. Additionally creating panels to fit between the grill shell and the radiator will also help to ensure the air goes through the radiator, and not spill over, under or around the sides of it.

As for sizing, perhaps the wise thing to do would be to contact a manufacturer, such as Walker Radiator, and ask them to size a radiator based on the engine you will be using. Then you will know 3 core vs. 4 core, etc. They make a living do this, see if you can get their recommendations.

Hope this helps!