Best? Your call, but i've used the flatening additive in the clear and i've used the rot rod black. The original is called "Organisol" I prefer the flatening additive from my experience.
Small Block
Ive used a few SEM Hot Rod back works well. If your painting in base/clear I think black base and a satin clear (usually used for trim) would work the best. No worry about water spots and you can wax over it.
I have used Rustoleum Satin Black with great results. You can buy a pint and put it in a spray gun and it will even give you a slight texture like organisol. Cleans up nicely, last a long time, and does not get water spots. It is what I used on my Pro Street Challenger in this pic.