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Here is a recent thread on the value of a 56 frame.
I suspect that a 56 frame is worth slightly more than a 61 frame, all other things being equal. 56 & 57 frames are different enough from later year frames that they are detectible (but with enough work they can be modified to look correct).
$1500 - $2500. They come up on EBay all the time. Don't see that they sell, though. In many instances they're rolling frames with complete suspensions and wheels.
Remember that in most states, the VIN follows the frame. And on C1s, the VIN is stamped on the frame on the top of the rail that goes under the driver's seat. That's well known in law enforcement, so that the law enforcement guy who doesn't know will find it when s/he looks it up. Make sure that the frame you buy has no clouds on its title. Best way to assure that is to get a title with the frame.
Then you'll have the problem of titling one car with 2 titles - one for frame "x" and one for body "y". It can be done, but your friendly and hyper-efficient Registry of Motor Vehicles will have heartburn with it. Their solution may be to issue their own (third) title and crudely affix it to your car. That would detract from the car's value.
Even if no titling is presently involved, you want to think long and hard about creating a car that cops will think is hot. You will be selling this car someday.