How should I connect to a short ground wire in a metal box?
The simple answer is get "self grounding" duplex outlets.  The typical design has a little spring plate meant to connect your grounded metal box to the outlet yoke, and thus to the third prong.  Just install, test the ground with an inexpensive outlet tester, and feel lucky that the builder left you in such good shape:



If you are in the United States: surprisingly these are legal even in new construction.  Beyond that, they're perfect for your retrofit.  See the National Electric Code (NEC) section 250.146(b). Note that your ground wires may 16 guage rather than 14 or 12. That was OK at the time, and you're not required to change it now.