you'll end up around .039 in the hole in a magnum based of a 9.58 height.. Also I've only messed with 1 set of magnum heads and they were 64 cc when I actually measured them. Here is the way i see it:
swept vol. 823
deck vol 8 (using -.020 for the magnum deck mentioned above 9.58)
valve relief 4
head gasket 9
head 64 (I'm using this based on my measurement of an actual head)
908/85 10.68 to 1