Given that he slowed down a lot with the heart coming out, and that you say he never check raises, my read on the river is that he was playing something other than a heart draw (maybe a small heart--maybe 66 again) like a pair, two pair, or maybe even a set. I'd value bet the river for about $10-20 depending on your feel for the situation. I think you're probably best here, because I don't see a maniac slowing down with a 1st 2nd or 3rd nut flush on the river after being agressive up to this point.

I'm eager to see how this turned out.

Edit: I forgot to include this in my original post, but why didn't you raise 99 preflop? I'm not sure if any information gained from a PF raise would help with this villian, but I'd be interested in your reasons for simply calling at a 6 handed table.