Some of the replacement engines didn't follow the same machining practices as the originals. When we built John's 261 with the "high mount" water pump location, we found the locating tabs for the main bearing shells machined like the late-production engines,which meant bearings we had on hand didn't want to fit. I did some careful drilling of dowel holes in the block so the "old style" main bearings would fit. Apparently, it was a block designed to replace the high water pump engines, but the lower end was machined for the later style bearings.
Jerry
