ZL-1 Camaro engine VS early Can Am engines

[GMH454]

My first interest in these was through Corvettes and racing. Greenwood used them, Warren Agor ran one in his Group 6 Camaro in Trans Am / IMSA etc. Jim Hall ran a type of ZL-1 in around 1967. There is a photo of the engine out of the car showing the prominent front ribs on the block.
I have always read that the early Can Am engines were different from the COPO and later engines. Does anyone know what these differences were. (Note not talking here about the Reynolds blocks that McLaren ran ) Have read that there are differences but never see anything mentioned.
Anyone know of any references to another site would be appreciated.