Might I suggest that the diameter of the inlet/outlets is correct for the design of the radiator and shouldn't be altered? If you just split a larger tube into two smaller tubes that are exactly half of the cross sectional area of the larger one, you'll flow just as well. Something like you'd see in an V8 exhaust header, where four small tubes merge into one. As the water flows through the larger tube, when it meets the split, if the lengths of tubing and the radiators are the same, the fluid should flow equally into each radiator, then they can merge back into the large diameter tubing and head back to the engine. Each radiator will have to have a bleed valve on it, but I don't think putting a large inlet/outlet on is the smart thing to do. This is only my opinion and I'm not a hydrodynamics engineer, but it make the most sense to me. If you get a decent airflow over those radiators and use aluminum on the way to the front and back for extra heat loss, I'll bet you you'll be just fine.
Jason
