The Stovebolt Site Forums The Shop Area Engine Shop 1955 Bearings, WTH : )

The casting number confirms it's a later 1955.2 low water pump position block, so the upper bearing shell should easily rotate out as Jerry said. Your stamped serial number (presuming one is present) will confirm what the engine was originally installed in. Check out the screenshots below for decoding info on the serial number. The first pic is for "truck" engine codes. The second is "passenger car" engine codes, as both engines used the same block casting number.

I agree with your assessment that leaving it as-is is probably a bad idea. A visit to a machine shop I think is in order.