Did this a few years ago on a '37 Tudor. As I recall, the regulator was able to easily fit into the wall panel through the larger opening. Then it attaches to the inner wall panel with four bolts & star lock washers. (a reproduction fastener package was available when I did this) The glass needs to be out of the way, lifted up or removed. Removing the glass ain't so easy.
The bigger problem, a tedious aggravating process, is getting the bottom glass channel to re-attached to the regulator.