I am not "hands on" with a '40, but here is hte usual deal. If you remove the windshield mouldings, you will probably find a series of screws across the top of the dash. There are also screws on each end of the bottom side of the dash. It seems to me that the screws along the bottom are "bolts", meaning there are nuts on the underside of the dash.
You can see a rear view of the gauges for a '38 here, four bolts at the bottom, 2 studs near the top: