Fascinating discussion.
I have two 460s (a 521 and a 557) with exactly the same click-click problem.
I have tried direct wiring to the starter from the battery, small Ford starters, geared starters - two different kinds - and no joy. Same problem.
I now know that if I try it enough times - say 10 - it will turn over just fine and start up.
One interesting sidelight: just after I install a different starter (and it doesn't appear to matter what kind just as long as it is new) the problem goes away for a while. Maybe a week, maybe three months. Then it comes back. I presently use the small Ford F-350 truck starters.
If anyone has a real diagnosis for this I'd be grateful. Maybe enough to send a bottle of really good single malt!
I've thought it was limited to the 460 engine family, but from the discussion above it isn't.
BTW both engines use automatics so no aluminum flywheels.
For the present, misery loves company
Tom