There is a manual available online that might be helpful.
This page shows the octane adjustment along with instructions for setting timing:
http://chevy.oldcarmanualproject.com...55ctsm0620.htm
Red arrow points to the octane adjustment in both photos. In order to adjust timing by the instructions you set the octane adjustment to 0, loosen the distributor clamp (yellow arrow in photo of your engine), and set distributor position using a timing light. Once the timing is set you can make changes for increased or decreased octane by loosening the bolt on the octane adjustment and turning the distributor.