For the speedo, you will need to use the output from the new PCM. Your 96 gauges need the PCM to translate the VSS signal into a usable signal based on gear ratios and tire size.
The tach will use the output from the PCM as well. The tune can be adjusted to work with the 96 tach which is looking for a 8 cyl output rather than the 4 cyl output the 99-up gagues want to see.
If your harness doesn't have PCM outputs for those 2 things, you will want to add them.