Guys, you're getting away from reality when you start talking emmisivity, paint colours, polished surfaces etc and radiator performance.
Fact: The emmisivity of a radiator will have virtually no effect on heat transfered. It is the forced convective method of heat transfer that does the job. Heat energy transmitted by radiation depends on the temperature difference to the power of 4(!), so the realtively small temperature difference between a radiator at say 90 deg C and the outside world at say 20 deg C means you can forget radiation and emmisivity effects.
I did some approximate calcs on this to demonstrate years ago to try and stop another waste of an argument, but can't get
AutoSpeed - The World's Best High Performance Online Magazine website to cooperate with my search. If you can sucessfully search for radiator and emmisivity in that web site then you'll find the calcs, probably!
Heat transfer from coolant fluid to air has to go through several 'resistive' effects. In order they are: Boundary layer of coolant, fouling on inside of radiator, radiator material, radiator coating (if any) and air boundary layer. Out of these resistive layers, the radiator material itself (either brass or aluminium) also is rather small in its effect.
So it's the design of the radiator that should concern you, not the material (except for weight of course!). It just so seems to happen that the latest and greatest radiator designs are in alloy, as you'd expect given the weight and cost benefits and car manufactuer constantly seeks.
As to how far over you should angle your radiator, dunno. Too far and you'll have trouble getting sufficient airflow however. In my experience, the more flow you can get through a heat exchanger, the more heat transfer you'll get. Well, that's what the formulas say, and what I found during 7 years designing industrial shell and tube exchangers anyway...