Well, I was going to be helpful and post a link to the Garmin page for the template. Oddly, the 400 flushmoutn installation guide can be downloaded but the 500 series cannot - it takes you to 'pay for' ordering page for hard copies. Must be a link glitch on the website.
However, the face of the unit measures 160mm wide by 145mm deep and any hole in the console won't be much smaller as there isn't an overhang lip - the flushmount bezel does that bit. You'll need around 60mm clearance at the back plus room for the connectors. Looking at the back of the unit, the centres of the sockets are approx 45mm from left in and 65mm down for the multiway and 45mm in from right and 65mm down for the antenna BNC.