Hang punching bag in finished basement
I wouldn't be bolting the supports over the drywall. You will get movement back and forth from the bag and that movement will start to compact the drywall and eventually loosed the supports.

Think about installing a 4x4 between two joists with joist hangers. Then drill a hole through the 4x4 and install a 1/2" eye bolt and hold it in place with nuts and washers.