Great info. Also you can do it the way I did on my buddie's '42 Willys pickup. Here too, the AC unit took up the entire area behind the dash. I made some cone segments of 18ga sheet metal with the apexes sized to accept the gauge body. Welded them to the dash panel, a little bondo and sanding and paint finished the job. Born totally of necessity, that dash is one of the most popular features of the truck!
Since yours is a fiberglass dash, the cones could be made with your wood/cloth buck system and bonded to the dash. A feature of the cones and one that could be easily added to your dash module is to angle the gauges for easier viewing by the driver.