Does the 1991 Chevy Silverado have an ACC position on ignition switch?

No, the 1991 Chevy Silverado did not have an ACC (Accessory) position on its ignition switch in the same way later models did. While it had positions for "Off," "Run," and "Start," the functionality of an accessory position was often integrated into the "Run" position. This means that some accessories might have worked with the key in the "Run" position, even without the engine running. There wasn't a separate, distinct "Accessory" position like you'll find on later vehicles.