Mike,
The casting number I used to identify my '54 261 block were located behind the starter and could not be read until the starter was removed from the bell housing. I used the following link from inliners to decipher my casting number.
http://www.inliners.org/becks/BCN2.html I didn't have any luck finding the numbers you posted on the list of casting numbers. Is it possible that you were reading numbers from a some sort of tag that was attached to the block rather than numbers cast into the block? It is sort of difficult to tell from your photos, but it looks as one of the shiny areas that you cleaned up could be a rebuild tag.
I would be careful about determining that the engine is a '261 solely based upon the presence of captains' bars because my '54 dosen't have them and I believe the 235s from 1954 did.
Good luck and keep us posted.
