deanyel is right about preparation. Speaking from experience, I built a GM 350 from the block up for my streetrod. I had the block hot-dipped, then did lots of grinding on the surfaces that show. This was followed with a liberal dose of metal prep, then high-build primer, sanding, and finally Dupont Centari. This is a common 3 part catalyzed automotive paint. You could go with base/clear, but that would be silly on an engine blok, IMO. And don't let anyone tell you that the heat will ruin the paint, since if your block gets hot enough to burn off the paint (assuming it's been properly applied and the surface correctly preped), you have other problems. Expect some colour changes around the exhaust manifolds, though. To do this yourself, engine in car, I'd expect that getting off all the surface contaminants would be a real challenge. And, unless you have a buddy in the business, there's a lot of money to spend on the paint itself, because the reducers and activators don't come in small containers, even though you could get a pint of paint to do the job..

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1986 560SL
2002 Toyota Camry
1993 Lexus