I had a discussion about this recently with William and learned a couple of interesting things.
I was told the oil line location through the firewall is a biggy. Usually never put in the right spot with the correct rubber grommet when gauges are added.
I'd also be looking close at the wiring harness. Factory gauge cars, like mine, and as I've been told by a couple of smart people here, should have cloth covering on both coil wires. An odd clue I had not heard until recently. Reproduction wire harnesses don't come with it.....I tried. I'd be crawling under the dash as well and checking the console and dash harness connections for obvious monkey business.
Of course as everyone knows it should have a clock as well, so many conversions done without.
This is exactly how I determined the gauges were NOT original on my '67. Even though they are original gauges from another car and a correct gauge option speedo/tach cluster from another car was added (prior to 1982) to make it all seem factory installed, the oil pressure line was routed through the firewall behind the master cylinder. The dash harness connections are not quite right either.
Edit: I went back and found the thread of this issue on TC to refresh my memory. If you're interested, here it is:
http://www.camaros.net/forums/34-restoration-corner/259425-oil-pressure-line-console-gauge.html