i'd listen to momma
radiators designed for passive vs. active cooling are designed very differently. passive cooling radiators are designed so the fins allow the air to move vertically and space them more openly because the natural boyancy of hot air moves upwards and fins that are too close to each other restrict this flow. and no, placing the radiator w/ the fins facing up instead of forward will not work. the fin density is too high. you will need to force air.
on a side note, you will more than likely need a pump larger than the typical liquid cooling pump because the pressure drop across a car radiator is enormous compared to a computer radiator.
i suppose there is the possiblity of the radiator being able to hold SO much liquid that it would take days for the computer it heat it up but... it would be touch and go.
you can take my word for it, im a convective heat transfer engineer.