Best way to remove nipples on water pump?

If its not too late to voice an opinion;

Heat expands, and cold contracts, these are the laws of physics...can't change these.
You're dealing with pipe threads (NPT's) that are tapered. if you can get any movement at all...you can get them out. A bolt , or anything solid, close fitting to the tube I.D. that gets down the tube approximately where the threads are (let this plug or bolt sit in the freezer until you're ready to use it), is great, but try this:

heat the casting, not the tube, because you want the casting to expand, with a propane torch, then drop in your plug or bolt

support it against some flat and solid ( two person job), have someone strike the top on the fitting lightly while holding the pump steady

try to tighten and then loosen alternating. Remember, if you can get any movement, you can work it out