Did some checking on the pistons that came with the block I picked up. Here is what the vendor response was. Any comments?

The H618P and H618CP are close to the same, the biggest difference between the two is the H618P calls for more clearance than the H618CP.
The H618CP has more silicone content in it so when it gets hot it will not grow as much as when it gets hot.
To set the clearance on the H618P30 you would measure down 2.100" from.the top of the piston and add .001".

Not sure what this is telling me.