The Cinelli is a pretty big frame, much larger than a 56. It's got some really nice parts on it though. If you're willing to do a lot of wrenching, you could buy a couple of bikes for the parts you need and sell everything else off. More work and more initial cash out of your pocket but with patience you could come up with a nice machine for a very low price.
I would want to get more information about the Cinelli frame. I'm not a Cinelli expert by any means but the head lugs don't look like any I've seen other Cinellis.
Sizing then was usually seattube measured center to center. Maybe around the late eighties bike companies started measuring center to top. The top tubes were usually close to the same length as the seattube but not always.