I’m in a similar boat. I just bought an original radio off of another member on here, and he sent the speaker also. The speaker is in very rough shape, but just even having one no matter the shape answers a lot of questions I had and gives me ideas for how to replace it. Attached are some measurements, if they help, of the original speaker. Because of the camera perspective, the measurements may look a little odd (at the widest point it is 5 1/4 inches), but you can get the exact bolt hole offsets from the dash and use this as a reference guide.
I am very interested to see what you come up with and how it works.