Chamber size??

Well how come the comp. ratio is so low in 74(8.6) compared to earlyer 68 318 (9.2)? Isint just because of the chamber size difference?

check out this article http://www.carcraft.com/howto/116_0401_318/index.html

In here used stock magnum heads from MP, and it says that they are 59cc chambers. And they decked the block and it gave them 10:1.

So im just trying to figure out what my compression would be if i put these heads on my motor but dont mill the heads or block.

BTW: i am not using Mopar Performance heads, i am using a set from the junk yard. Thats why i dont know if the chambers are different.