Presuming that your car is fairly standard and not highly modified: rear seat belts were not required on new cars until 1 January 1971 (as per the tables in the document from VicRoads), so you don't require them because your car was built in 1969.
If you do fit them (or anchor points to fit child seats), there are rules that you need to follow (eg location of holes and reinforcing plates to hold the bolts) and standards that you need to apply (eg using new, approved seat belts). VicRoads would have a document of some sort that either explains the requirements or refers you to the relevant national standard. (Found it:
http://www.vicroads.vic.gov.au/Home/SafetyAndRules/SaferVehicles/VehicleDesignStandardsAndAccessories/VehicleStandardsInformation.htm)
BUT if your car is highly modified or you do significant engine upgrades (eg turbo) then you might be required to fit seat belts as a compulsory safety upgrade. NSW requires this but I have no idea about VIC - some things vary between states.
If you want a qualified professional opinion, contact an authorised automotive engineer - VicRoads has a list of approved engineers. (see the VASS list link on this page:
http://www.vicroads.vic.gov.au/Home/SafetyAndRules/SaferVehicles/LicensedVehicleTesters/VehicleAssessmentSignatoryScheme.htm)
Otherwise, have fun and post up some pics in the Members' Gallery!
(edited for clarity and added links)
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