Are you able to roll the upper bearing shell out of the block? There are two designs of upper main bearing shells- - - -one has an anti-rotate tab at the parting line where the cap meets the block, (1955-62) and the other (1954-early 55) has a locating dowel similar to the lower bearing in your first picture. The later model upper bearing with the parting line tab can be rolled out and replaced by loosening the other main cap bolts a couple of turns and tapping the bearing shell out. The doweled upper bearing requires the crank to be lowered almost an inch before it can be removed.

If the upper bearings can be removed, you might be able to replace all the main bearings without a full teardown, after doing a "shoeshine" polish on the crankshaft with a strip of emery cloth. Just be sure to do a thorough cleanup of the sanding grit. The doweled upper main bearing sets are almost impossible to find these days, particularly in certain undersizes.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!