My original radiator is out of the car (1974 Dart Sport), and I want to clean it up and repaint it. I'm thinking about sanding off the old paint and wonder what kind of paint to repaint with (flat black, etc.). I redid my valve cover and I'm not really happy with the appearance. Has anybody done this, and what did you do to make it look NOS? Do I use a wire brush or paint remover, etc?

Standard engine paints are too thick for radiator use because they can interfere with heat transfer and block air flow between the cooling fins.
So use radiator paint. Pretty sure you can get it at most retail auto parts stores.
As far as color. that's personal. I like black satin finish.
I use whatever I have laying around. Carb cleaner gets the oils off and leaves it nice and dry for painting. Make sure to use the paint in 3 light coats rather that spraying it on thick in one coat.

HERE'S an example. There are also gloss versions.
A thick coat will reduce heat transfer.

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"When you find a big kettle of crazy, it's best not to stir it." - Pointy-haired Boss

1964 Valiant V200, 225/Pushbutton 904
BBD, CAI, HEI, LBP, AC, AM/FM/USB, EIEIO

Okay, I ordered the satin finish radiator black Eastwood paint. Is there any special way to remove the old paint before applying three light coats of the Eastwood paint? How will it compare to the original radiator?

Steel wool, brass brush, sandpaper, water. Be really careful about going at soldered parts too aggressively. You really don't need to get the whole thing down to bare metal. Just get all the corrosion that you can so it doesn't look like you painted over dirt. You'll be suprised what a difference new paint will make. If you have bent and ugly fins you can buy a fin straightening comb too.
Of course you could always take it to a radiator shop and have it boiled. But that would be getting out of the old do it yourself and save some money spirit.