My Blitz's

[Grant Bowker]

The screws holding the back plates onto the pump body are usually staked in place against vibration by using a punch to drive some of the metal from the plate into the slot in the screw from the edge. If they overdid it this could be adding to your trouble. Heat should soften this as well. They might have used loctite or similar against vibration - heat tends to break that bond too.
I can't think of anything in the top corners of a Chev water pump that should be hurt by judiciously applied heat. Don't go so crazy with the torch that you melt the casting but you should be quite safe short of that.
You may be able to get enough heat from a propane torch as used for soldering domestic water pipe fittings but oxy-acetylene lets you put in more BTU/min and keep the heat more where you want it instead of spread so far through the whole casting (and perhaps cooking the lubricant in the bearing), possibly actually using less heat than the lower rate torch.