Plan A was to install directly to the floor. In fact, I drilled the holes and attempted to install the screws. But that thick pad beneath the carpet was a pain. First of all it made it hard to locate the holes in the metal. But more significantly it's so compliant I didn't think the radio mounting bracket was going to be very well located.
So I followed devlish's advice and mounted to a scrap piece of plywood I had lying around. It was only a 1/4" thick piece. Top grade stuff, but starting to check from lying around without ever having been sealed. I used the four screws that came with the radio. They're for sheet metal but looked fine for wood. If that had been plan A I would have gotten six or eight wood screws, as the the bracket had plenty of holes.
Here is the result:
I ran a ground wire from the mounting bracket to the front left seat mounting bolt. I scuffed the paint on the bracket with steel wool. The wire at the seat mounting bolt is bare and tinned. You can't tell from the photo, but I routed the wire up through the hole in the seat frame, so that it's pinched between the seat frame and the lower facet of the mounting bolt.
In retrospect the rear, right seat mounting stud might have been a better location. My Lido LM-300 faceplate mount still hasn't arrived. It attaches to the front left seat mounting bolt. I'll probably have to redo the ground when the mount arrives.