Did you say a "Bronze" Shaft???? And you have a steel
boss over the bronze shaft?
Hmmmmmmm, you've got fun haven't you.
Hylyte has great advice. I would also attempt using a hydraulic bearing/pulley puller. It's going to take some force and I doubt a standard puller would do it. As yoyu apply the force, give the back of the coupling a crack with a heavy hammer onto a Bronze drift. Apply penetrants as well and even leave standing for a day or more till you have another go.
Heat would be your best freind. If you can get a Gas torch down there, heat the collar as fast as you can and keep a wet rag around the bronze shaft or even have water
dribble over the shaft. You need a lot of heat so as you can sink it qwickly into the collar before it has a chance of getting to the rubber. Then as soon as the rubber gets hot, you will have to stop and quench the whole lot real quick. Even the heating
and quenching may unfreeze the joint. Don't attempt heating
unless you have a good tip size. Those small hand held cannister thingies simply won't do it. You will do more harm than good.
Keep us posted with the results.