flushing out filings??

I rather doubt flushing a million gallons of water through would actually get all them gremlins... though it'd be a good idea to try maybe 100 or so.
The problem is  when they get trapped behind something now , they won't go free until you've bonced the tractor 1,000 times in the field.
Filings should be small, only place I can think they'd get trapped( once free) would be the rad. Maybecross fingers, then 3 months from now, backflush the rad, see what comes out ?

Too late now, but when you run the tap thru. pack grease into the tap and the chase the threads. The grease traps most of the pieces of steel.

Just a thought.  Because of the way the twist works on drills and taps, most of these little metal bits come out rather than fall in.  As small and "bright" as they are, I would imagine that rust would reduce them before they can cause any real problems.  If it helps you sleep at night, take a similar piece of cast and drill through it to see how much and how small the are.  For the dedicated "worry warts"  I would recommend getting some of those high power earth magnets ( at hardware store or internet) and place them on one or two "freeze plugs" to immobilize them.