To be sure:
* Consult your owner's manual: If you have the original owner's manual, it will show the exact location.
* Look for markings: Carefully inspect the engine compartment for a reservoir with relevant markings.
* Use a parts diagram: A parts diagram specific to a 1968 Chevy Impala (available online from various parts suppliers) can pinpoint the location.
* Search online for images: Searching for "1968 Chevy Impala power steering reservoir" with images will show pictures of engine bays, which can help you locate it.
Remember to check the fluid level with the engine off and the car level. Add fluid only if it's low and use the correct type of power steering fluid recommended for your vehicle. Consult your owner's manual or a reputable parts store for the correct fluid type.