Mike B brings up an important point. When you have a seeming mismatch in components that don't seem to go with the casting numbers .... You *must* check the date codes!
For example, my 261 had casting numbers that told us it was a '54 engines. But when we opened up (*after* I bought '54 style main bearings) we found it had the later style bearings in it. Checked the date codes and we found out the engine *had* come out of the '54 mold ... but in '59. Meaning, it was a crate motor built as a replacement for the later Advance Design trucks (before they went to the Task Force design that required the lower mounted water pump). At some point after '54, GM changed the main bearing design.
Point being -- when doing ID forensics, you have to cross check all the data you have, including the date codes. They *might* help you make sense of it.
Then again, they might not... Still, you should check the date codes.
