I currently own a '70 Monte Carlo and I know that most of the mechanical parts (suspension etc.) are interchangable with a '70 Chevelle. I'm looking to buy a '66 or '67 Chevelle and was wondering if the 66 / 67 have similar interchangability with the '70. Are the rear end, front suspension, brakes etc. from the Monte interchangable with the earlier Chevelles?

I am not too sure. But my best guess is not likely, as the 66/67 was a different chassis and body design. The '70 Monte shared the same platform as the 68-72 Chevell's

We use a Hollander interchange manual to make those decisions. Your local salvage yard should have one if they have been in business for any time. Or purchase one for your own personal use.