The job is done at this point but the thermostat is well out of the way not accessible for easy removal. Even if you pour the coolant "straight through the block" it's going to take the path of least resistance and leave many channels untouched. Maybe with a pressure flush you could get most of the passages mostly cleaned out, but I doubt it. Seems to me the best thing is to get clean coolant in there and let it do its job over time, then repeat.Maybe I'm not understanding the coolant system layout of your vehicle, but on all of mine, I've removed the thermostate and the larger block to radiator hose and you can flush and pour the new coolant straight through the block. Obviously reattach the bottom hose, fill with hose water, rinse & repeat until you are satisfied, drain again and switch to the distilled water mix.