The junction between the fins is the only place heat is transfers and a copper brass radiator the fins are soldered with lead based solder and that is a worse conductor than copper/brass so it about evens out, but the copper/brass is still more efficient.
There is nothing wrong with aluminum radiators, if you built a copper/brass with the same design as an aluminum it would be a little more efficient but the added efficiency might be offset by the added weight.
As for copper /brass radiator makers
US radiator is a us based rad builder. They can customer build a rad that will meet your needs
