There's a good reason for the difference in the bearing design. To change the upper bearing shell with the dowel built in, with the engine installed in the vehicle, it's necessary to lower the crankshaft nearly an inch to allow the bearing to roll out of the block. The later model bearing with the anti-rotation tab at the parting line and no dowel in the upper bearing shell can be rolled out by simply loosening all the main caps a little and putting a cotter pin into the oil hole in the crankshaft. Since the spray-oilers didn't have holes drilled in the crankshaft, the doweled mains didn't make much difference. GM also figured out that adjusting main bearing clearance with shims was not really necessary with the precision machining procedures available in the mid-1950's.
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!