8.5 sounds about right to me for 1.88"/1.6" valve J heads without metal shim gaskets if the motor was about 9.25:1 with magnum heads.
Most people fit the J heads with bigger 2.02 valves which makes the difference much less. In the above example it would be about 9:1 with standard gaskets and 2.02" valves.
My J chambers come in at 66.5cc with 2.02 valves, with 1.88 they came in at 70cc
Magnums are about 63cc with a 1.92 and 1.625 valve.