First thing you must do its totally block off the large hole on the heater box where the pipe between it and the vents go, otherwise you lose loads of heat behind the dash, mine wasn't blocked off when I bought my car, was a cold day and the heater was crap as all the pressure was being lost though the vent output on the heater box.
I don't know what people do to secure the gauges in the hole, but I would most likely either make some small metal brackets to secure each side, or use some mastic to stick some wood blocks under the edge of the metal plate and push in till snug fit.
tabetha