I'd have instacalled.
After tanking and his showing Kd I'd be really confused. Firstly confused about why I'm tanking
Scondly confused about the Kd.
I honestly don't think someone with king high flush overbet all in much less then show the king. Of course that would make it an epic play if he had it.
Therefore I call for two reasons:
1) I think I'm ahead.
2) if I'm drawing dead this guy deserves my chips for such an outrageous play and I want to see it.