[19Fordy]

My 40 doesn't have a radio but, I had a similar problem when I wanted to mount my Constant Voltage Reducer behind my dash without drilling any holes, welding that would damage the paint, or alter the dash or surrounding metal in any way. To solve the problem, I made a mounting bracket out of .090 aluminum sheet metal for the CVR and mounted the bracket using the 2 existing stock holes inside the bottom of the dash below the 2 dash knobs you see in the photos. The thickness of the al. sheet metal allowed it to support it's own weight and that of the CVR without flexing. Hence, the 2 screws with nuts is all that was needed. You can't see the CVR because it's on the opposite side of the bracket. Perhaps you can adapt this idea by bolting a small speaker to the back of a piece of aluminum sheet metal. If you do any welding on your dash you will bubble the paint and open up a can of worms. Good luck.
I think you can get a pretty small high quality speaker that will fit.