Chamber size??

after 71 they dropped compression by moving the piston pin up. this dropped the piston way down in the bore about .100"

The earlier 318s that are rated in the 9:1 are are lower then that. probably in the *.5-8.8:1 range.

The real bad news is your 318 is likely in the 7.8:1 range.

The only way to know for sure what your compression is ,is to measure everything. Head CC's, Deck hieght and gasket thickness.