Useing the wallace calculator, with a .041 head gasket and a "0" deck height, with 6cc valve reliefs which is common for eyebrow pistons it comes to.
Your displacement is 406.77 CID
The Compression Ratio is: 11.53
course there are lot's of factors to change this as mentioned in the earlier posts, such as deck heigth, difference valve reliefs and different gasket thickness.
JMO
Zip.