What size is the gauge? Mechanical or electric? Where it goes - the tap in for the gauge or the gauge its self?
As mentioned, a good tap in location is the T fitting under the turbo in the side of the block. I removed the fitting, drilled and tapped it instead of using another T fitting - much cleaner installation.
As far as where to mount the gauge, there are several nice options for 2 1/16" gauges such as the A-Pillar mounts or the T-Top mounts.