Get a good radiator shop to do a flow check and get what you have to base line - Old water pump? then you can build from there.
Don't buy into the aluminum radiator myth. don't forget, the weight of a radiator is a lot to do with the weight of the liquid coolant. beyond that, a dry aluminum radiator is not that much lighter than a dry copper/brass radiator and the both reject heat at nearly the same rate - given current design core design and a fresh un blocked core.