The late model main bearings (56-63, I believe) still have a dowel- - - -but it's only on the lower bearing that fits into the cap. The upper bearing shell has a tab at the parting line that fits into a divot in the block instead of a dowel, so an "in the frame" main bearing replacement is easier. The outside diameter of both bearings is the same, so buy the later bearings and use a 1/8" wide grinding wheel in a Dremel tool to make a slot in the block for the tab. It's about a 30 minute job for anyone with a few brain cells to rub together. Your "machinist" may or may not qualify! It's still possible to adjust the oil clearance with shims between the main bearing caps and the block, even with the later model bearings installed.
Jerry
