I know the thrust block in the '57 and later open-differntial axles is very stout. The sure-grip differentials used a weaker thrust button that was two halves held together with a flimsy brittle roll-pin. Not sure what is in your '55 axle. I would be more concerned about damaging the axle threads or wheel bearings. I would use as little heat as absolutely possible as temperatures of 350-400degF start tempering the heat treatment out of parts. I would focus on rigid tooling and a good solid whack to separate the parts. The more rigid the tooling, the higher the resulting force is from your impact. Force is what separates the parts - its the Laws of Physics. I have had much experience with keyed tapers as well as newer keyless taper designs used in transmissions having capacities up to 3,000 HP (diesel).