There are two types of radiators. Copper and Aluminum. Aluminum is generally thought of as the better material, but many people have had success with copper as well. If your radiator is old, chances are it doesn't matter which material you chose as your old radiator is probably clogged and any new one will show improvement.
Size
The old timers rule of thumb for choosing a radiator size to to match the square inches of fin surface with the number of cubic inches you are trying to cool. The radiator for a chevy 350 should have 350 square inches of surface area, i.e. 35x10 or 21x17 or whatever. It's a good place to start. Most of the time, the engine bay will be the limiting factor on radiator size.
Cores
Coolant runs through a radiator is layers of tubes called cores. The first core, toward the front of the vehicle, is the most effecient one as it recieves it's air from outside the vehicle. The second core recieves its air from the first core. The air is already warmed by then, so it's not as efficient. succeeding cores decrease in effeciency by a given precentage until the air temp matches the radiator temp. Genearlly, more cores are better. A 2 core radiator is more effecient at cooling than a 1 core. The problems that come with hight core count are weight and air resistance. The weight differences are negligible for rockcrawlers. Air resistance is a bigger concern.
