best big block?

[Hank70SS]

No replacement for displacement, so if you're after horse power it's the 572. It's not cheap and it is a tall deck. So you'll have a harder time with headers and other accessories. Also needs an electric fuel pump and doesn't have a boss for mounting a a z-bar if you have a stick. It's also easy to destroy the rest of the drive train unless it's upgraded as well. Let's face it, a 66 wasn't designed to handle that kind of horse power.

The 502 is not a tall deck but also not set up for a mechanical fuel pump or stick. I think it has the mounting boss for a z-bar but needs to be drilled and tapped.

A 454 with 500+ hp can be had for about half the price of a 572. Even a GMP ZZ454 crate engine, 440 hp and 500 ft/lb torque is a good option. It's not crazy hp but will move a 66 pretty damn good. It also is a roller cam setup which is just more reliable than a flat tappet cam.

If you want a cam with a nasty lope then look around for a custom built 454, go here,
http://www.lewisracingengines.com/
or here,
http://www.vortecproperformance.com/

They both build 454's with some pretty impressive numbers and reasonable prices. Tell them what you want and what you're willing to spend and they can put something together that you'll be happy with. They both have very good reputations, before and after the sale.

Seriously, a 572 is the ultimate but it can be too much for a Chevelle unless the rest of the car is up to it. With a 572 and sticky tires you'll twist your frame, destroy the rear end or drive shaft or worse, kill yourself or somebody else. Don't forget, that 66 is now 46-47 years old, how much do you think it can handle?

EDIT: Didn't see your second post when I replied. I still say a 454, they made millions of them, parts are easy to find, new and used. 572, you may be able to find one but you'll pay twice or more for it and parts will be harder to find and more expensive. Personally a 454 is the perfect big block engine for a Chevelle.