What is the reason for wanting a 'Crossflow' rad rather than a vertical rad. ?
As far as I was always taught as water cools it drops and therefore naturally flows in a cyclic way around the cooling system. In fact my first car didn't even have a water pump it relied on a system called 'Thermo-syphon' i.e. hot water entered the top of the rad, cooled and exited from the bottom. It then got heated by the engine, rose to the top of the system and the whole cyclic motion started again - 'simples'.