Here's my recomendation. Whether you are doing a flush or not. don't drain the coolant yet. Put a Drain pan under the freeze out Plug area. Drill a hole ( about 3/8ths) to put a screw driver in and leverage the Freeze out plug off the Block. That coolant will help push the metal filing from the Drilling outside the Block.
Some Freeze out plugs are pretty thick, 1/16th of an inch, so don't push the drill too hard but don't think it's a piece of tin either.
Putting the Heater in isn't all that easy so , get yourself as much room to work as you can. If the Starter looks like it's kinda in the way, just get it out of the way and make life easy. On my JCB, the Alternator looked clear so I drilled and got the Plug out. After getting frustarted with the 'O' ring not cooporating, I got the Alternator out and with less of a dance it finally went it. When it does sit in, all of a sudden, you wonder what the heck the problem was on all the other attempts.
Good luck