For those of you that do not have in-dash gauges, here's a trick I used on my old 65 GTO as well as my current 69 Firebird. I have "idiot lights" in the dash, and I wanted aftermarket gauges to monitor engine functions. At the same time I also wanted to keep my original interior looking factory stock. The answer? Gauges that tuck up into the dashboard when not in use.
I purchased a 3 gauge mini-panel set by AutoGage. Each gauge is a mere 1-1/2" in diameter. You may use other brands if you wish, but the size is critical. The smaller the better, unless you have a mammoth cavity area in your dashboard. The AutoGage set is only about $45, and includes the engine temp, oil pressure, and voltmeter. The end result here will be that your gauges will be hinged, and will swing up into your dashboard when not in use. Whatever gauges you decide on, make sure they're electric units (not mechanical) for safety's sake. The tubing that is used for mechanical gauges is not designed to be bent repeatedly.
Locate an area under your dash that can accommodate the gauges lying "flat", or so the gauge faceplate faces the carpet. You want enough room in the dash cavity so the gauges will be totally hidden from view when in this position. Make sure the gauges won't interfere with your radio or other components. Now purchase a couple regular steel hinges (about 1 inch long), a kitchen cabinet door magnet (which should include the striker plate the magnet sticks to), and a 4 inch, angle shelf bracket. You can pick these items up at any hardware store.
Attach the two cabinet hinges to the top of the gauge panel. The idea here is that the whole gauge panel will be hinged, and will swing up into the dashboard when not in use. You'll have to do some experimentation with this setup to get it just right. To mount the hinges to the underside of the dash, you can probably use some existing underdash holes, then space the hinges outward accordingly. The last resort (in my book) is actually drilling holes. Once mounted, you can hook up your sending units, wiring, etc. as per the gauge manufacturers instructions. Now you must find one more underdash hole to mount your 4" angle bracket. At the end of this bracket, you will fasten the magnet using the supplied screws. Now you can attach the magnet striker plate to the gauge panel housing using heavy-duty double stick tape. Position the angle-bracket such that when you swing the gauges up, the striker plate will hit the magnet and hold the gauges in the "up" position, or such that the gauges are tucked into the dashboard. The gauges will naturally hang down while your driving the car, and when you park it, you simply swing the gauge panel up into the dashboard, and when you hear a "click", your gauges will stay tucked up. When your ready to drive away, simply reach over and gently pull the gauges down. Actually, even if you forget to pull the gauges down, the first little "jolt" you hit in the road will swing the gauges down automatically. Now when the car is parked and the gauges are tucked up, your instrument panel and dashboard will look factory stock. When the car is running, you now have gauges to monitor engine functions.