Jason,
Use real radiator paint put on very thin, or nothing at all. It doesn't take much to hose up the heat transfer. Ask a radiator shop what to use.
As far as the other stuff, a high temp paint or engine enamel will be fine. I haven't used the POR hardnose stuff before but the regular stuff doesn't stick to bare metal very well.
Whatever you use make sure you get all the oil off before you paint. Oil pans can be hard to paint. Bead blasting doesn't get the oil out of the pores in the metal. I would recommend at least a 2 day long soak in a caustic solution or a good steam cleaning followed by a "metal prep" phosphoric acid wash to get the oil out. Red devil lye in a 5 gallon plastic bucket works well to remove all oil and grease from just about anything metal. You can leave it in there for weeks, just don't get any on your skin.
