I believe the Optiplex 960 SFF uses a proprietary sized PSU. You likely can't replace it inside the case you have.
You can move everything to a new case to support 'normal' PSU standards, but Dell Optiplexes (unless something has changed) use proprietary motherboard connectors, and secure the heatsink directly to the case itself (no backplate) - the boards also tend to be 'non-standard' dimensions.
Those are major hurdles and it's rarely cheap &/or easy to make those things work.