I don't understand how the one wire of the harness that is the RPM signal is too short.
Is the whole harness not reaching?
If you're just having issues getting your hand back here try this.
Set the steering wheel to it's lowest point on the tilt. Hold the cluster in your left hand, between the steering wheel and the dash with the left side of the cluster in your hand. If you hold it strait up or at a slight angle the harnesses should reach. Once the harnesses are plugged in, rotate the cluster down and attach the speedo cable. ( Go under the hood, remove the clip that holds the speedo cable in the VSS, and oull it out. This will allow you to pull the speedo cable toward you to make it a lot easier. )
Once the speedo is clipped in, push the cluster in, pop in the 4 screws to mount it. Then put all the trim pieces, switches, etc back in. Dont forget to go back under the hood and put the speedo cable back into the trans and install the clip agian.
If this doesn't help, I'll try and walk you through it over IMs or some other means of communication.