Jeff-
Not sure about the differences between the years in terms of performance. However, there is a difference between the thermostat external bypass and internal bypass engines and heads.
My 1950 block is the external bypass type. The internal bypass types have an internal passage in the front of the block and a corresponding port in the head that replaces the external bypass system.
I think the Edgy head can be used on either, but the internal bypass port has to be drilled in the head to match the block. In this pic, you can see a slight bump on the left end, which is the front end of the head. This bump out in the face of the head is where the internal bypass port is located on the later blocks.
Food for thought.
Pete
