The pic shows the sizes. from the views you show your slots might allow a slightly larger dimension on the .750 number, possibly .760" to .765" if put into the slots and can work through neutral, I would do it. I found the pic from Neblink, but I don't know who took it.
The trans looks a bit rusty. as such the detent balls and springs can rust, stick, or fail. Using a space heater to warm the trans for a day or two (safely!!!!!) might reduce the moisture, Changing the fluid might be a good idea as well. 85W140 gl4 or gl5 is fine. 85-90 is also good.
The loose rim on the clamps do not often reveal obvious problems. They also often need to be loosened, lubed, and retightened due to stuck threads. Jim