To mount the frame under the dash, I used 5 minute epoxy GEL (it doesn't run like regular epoxy) and lathered it onto the top of the frame like icing a cake. I let it flash for a couple of minutes before sticking it to the underside of the dash...
After that, I wiped off any excess epoxy in the grill opening and did my final assembly for the speaker mounts. I'll seal off the holes for the wires on my next day working on the truck.
The bottom two pictures show the final assembly bolted into place. One is from the bottom and the second one is the best side-view photo I could take...