+1 for painting the top tank. Aluminum is a poor substitute for copper in the heat transfer department, but a new copper radiator will make your checkbook choke! Copper is the world's best heat conductor, while the only advantage aluminum offers are lower cost and lighter weight. If you're not having overheating problems, just paint the top tank and drive on. I'd recommend semi-gloss or flat black paint.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!