I don't think an engine block will flash rust fast enough to be an issue. The rust he is dealing with is not simple flash rusting.Look you need to hurry or else your going to chase your tail. Once you start removing thermocure / flushing it out , the inside will begin to rust. You have to do this fast or in a day because overnight it will rust again.
Ask me how I know, just go ahead
you got to get your coolant in there sooner than later or keep it empty until you can replace the parts and refill