You might also try power flushing the engine. I made a couple of adapters that connect to the water pump outlet and the lower inlet using pvc pipe fittings, then attached a garden hose connector. Connect to a hose and let ‘er rip. It was amazing how many large chunks and flakes of crud came out. Flush it both ways, several times. Whatever rust converter you use will be far more effective if it doesn’t have to work on chunks and can concentrate on the stuff that doesn’t come out easily. Obviously, don’t connect back to the radiator until your flush is particle-free.