The Autometer 5" gauges are not 5" they are less. Even the outside of the trim ring is less than 5". In the second pic someone put the 3 3/8" Autometer gauges inside the stock trim rings of a Firebird dash panel.
The opening necessary for an analog 5" Autometer is 4 9/16" so you'd need to enlarge the opening by a little over 1/4" all around.
If you're interested in the Autometer gauges, go to their site and pick a gauge. On the page for each particular gauge there's a link to a page with all the dimensions for all of the various models they sell. They're not exact though, the dimensions I listed above are exact for the cobalt series.
