The "best fit" situation would be a 1954-early 55 235 block, the one with the high mount water pump location. That puts the fan in the right position relative to the radiator. Yes, the slots for the locator tangs on the 56-62 main bearings would need to be cut, but any machine shop that doesn't want to do that doesn't need your business anyway.
Here's another option- - - - -leave the original main bearings in place, line bore (or hone) them to fit the OD of a set of Chevy LS main bearing shells, and grind all four crankshaft journals to the same size as an LS crankshaft. Crankshaft thrust is still controlled by the original #3 main bearing flanges. Main bearings for a Chevy small block 400 can also be used that way. Here's you tube video that explains the process better than me wasting a few billion electrons on text:
Chevrolet Straight Six is a One-of-a-Kind Creation!Jerry
