Steve, I think the whole flush idea was unnecessary anyway. If there were debris capable of clogging the radiator it would of been still clogged in the last one.
From your pictures it looks to be a nice original car. It also looks like it has needs that needs to be addressed if you're intending to drive it. When is that heater control valve
going to give up the ghost? It's nonfunctioning anyway with that split vacuum hose.
Not that it helps you now, but a garden hose could of been connected to the pipe where the elbow is connected below the thermostat with a piece of scrap heater hose.
If you hit that gas line with the torch while you're heating the broken bolt I hope you can jump quick.
It's quick work to remove the water manifold on a non-AC car to have at it with that bolt.
Have fun, this is a hobby 👍 Steve ROA9236