Tiny,
Without being 100% sure I understand what's going on, would this be worth a shot.
Somehow, whether by running the car in reverse while on stands, or with the cable pulled on the transmission and a Makita drill running the cable in "reverse", loosen the knurled nut on the back of the speedo and wiggle/pull while all this is going on.
My thinking is that the little square or hex that fits into the back of the speedometer might have made it own little retaining groove over the years. Wiggle wiggle, pull pull!
Stuart
