Here's an update on my earlier post. I also discovered that there's a rear factory speaker, so more details on that later! My true need is for the front speaker that mounts behind the dashboard (but, towards the front of the car). When the dashboard is installed (likely a windshield out job) the speaker is effectively buried. Also, the speaker fasteners seem to have a modest role in keeping the dashboard stable. This arrangement on the 2000 series coupe is unlike the 2800 coupes, in that the slats are an integral part of the dashboard face, as opposed to being a separate slatted cover.
See photos below. The length of the mounting ear holes, on center, is 17.5 CM. The height of the mounting ears, on center, is 10 CM. The total length, out to out, of the speaker cone is 15 CM including case surround, and the total height of the oval is 9 CM. The depth of this speaker is between 4.5 and 5 CM, which may be critical since the dado'd speaker slats in the wooden dash are so fragile. I would not want to put any additional pressure on the dashboard slats by trying to mount a speaker that's too deep.
Thanks very much for your help. Robert