Brian:
Make your dash then on the top flat surface where your gauges etc are to go that you made out of plywood and covered with FG, Cut it out about a 1/4" to 3/8" inside the out edge. Put your holes for gauges into that. Then put 1/2" to 1" blocks around under the lip edge where you cut out but hold those down 2 thicknesses.Now drop in the piece you cut out and secure with screws. Pull the screws now that you know it fits and cover with vinyl or paint or cover with SS etc drop in your gauges and hook them up. once done you can just screw it back in place or take out to repair problems or lights out.
Dave