radiator. what should i shop for?

so my dart will eventually have a mild 360. i have no clue what i should be getting, radiator wise. i know 0 about radiators other than they cool the motor haha! so lets just say the motor has 400hp(it wont, but lets just say..) what size/kind of radiator would i want? auto trans.

also, my uncles 73 charger's radiator is bad an need to be replaced. what size radiator would he need. is it worth the extra cash for an aluminum? its just his cruisier, not dragin or anything like that. its a 400ci completly stock with an auto