Even if it would be a '56 engine, it should still have the number stamped in the same place. It should be visible on the driver's side head, on a machined pad between the front and center exhaust ports. '55s started at number V400001. I'll be what happened is somebody switched the passenger side head from another engine. The original head might have cracked. Heads are interchangable as far as I know, at they are on the later 394s. I can't explain why you're carb won't fit.
Paul
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Paul Hartlieb
Membership Chairman
National Antique Oldsmobile Club
www.antiqueolds.org