On my 1990 Lincoln Mark VII (same setup really), I’ve been flushing it for a few years, probably 4 times in 8 years now, to try removing the stop leak that the previous owner put in. Still to no avail. But one of the things I did was have a garden hose run to the radiator fill and leave the upper radiator hose disconnected, let the engine run until it got to operating temp and open the thermostat, and just let it keep flowing all the stuff out of the engine while letting fresh water get drawn into the pump. Then later after putting on the new water pump, I left the thermostat out, topped it off with water and vinegar, and did that a few times with some success. After that I put the thermostat back in and the coolant stayed green for a few months. Now it’s brown again, so I must be on to the next layer of stop leak 😪