The fact the board paired, and our flush isn't great, and he just donked 400bb into us on the river makes me want to fold. Also from experience at 10NL / 5NL, 99% of overbets are for value. People will rarely do this as a bluff, they would probably go smaller if it was a bluff, like $9 so they can fold. I'd be inclined to say he had a good hand and was trying to get tricky, possibly boating up on the river.
The fact it's BvB and you have history makes me want to call.
But common sense prevails and i'm going to say this is a fold, would be interested to hear other peoples opinions though.
Oh yeah, well played IMO.