There isn't any info on what size chambers the 360 castings have. As they all just say "Open" which leaves a large margin.
If the chambers are around 116cc's you'll need a piston with a dome on it around 16cc's to get the compression ratio to the 9:1 spec as recommended for your cam. Since chevy used various chamber sizes on open chamber big block heads unless you measure it or someone else already has it's really a shot in the dark, as they could have 122cc chambers. If they do then you would need a piston with around a 22cc dome.