I'm in the process of cleaning out the block (225) and replacing the water pump and noticed this cavity in the block (pointing the screw driver at it) is this a water passage filled with crud or just part of the casting? I see that its not connected with the impeller area or passage below but just wanted to double check. I didn't see this on another photo of a slant block. I think the block is a 68 or 69

Normal and found on many SL6 blocks.
The correct water pump gasket seals-off that "cored-out" area, to prevent crud build-up but a bead of sealer will also do the job.
DD

Thanks DD! This is m first slant but its been a lot of fun learning about these motors on this site.