Household water pressure (typically 30-60 psig) is way too much to test a heater core. My ‘59 235 runs a 4 lb. cap so testing much above that is asking for trouble. I have tested filling the core with water, plugging one side and applying air pressure to the other. if you can get your air compressor regulator down to 8-10 PSIG, that is more than enough. You also may be able to borrow on from your FLAPS.
