I think all 305's are 9.5 to 1 with the 58cc chambers, as all 305 heads are. a set of 49cc chambered heads would bump to 10 to 1, give ya more power, you could get away with 89 octane saftly. if really have an 8:1 motor, than the 64cc heads would give you approx 7:1, in affect decreasing power. the 49cc heads seem to be your only liable option other than stock reworked, without a full rebuild