copper as a material conducts heat better than aluminium, but a alloy radiator can be lighter for the same cooling capacity. Also the effectiveness of the heat exchanger depends on the tube design and construction, modern designs all use aluminium for the weight saving and therefore newer designs probably have better efficiency than old copper and brass radiators.
Your proposed design could work, but I dunno to me I would really rather have a single radiator if at all possible. So the oil cooler in the pic is forward of the chassis rails? i.e. the radiator cant take the space where it is and be behind the chassis rails? how much room did the original radiator take up?
Also I dont think you will have to worry too much about the weight causing sledging, the rover v8 isnt significantly heavier than most iron block 4cyl engines, it's probably lighter than quite a few even.