loonychune... villain is 31/3 which usually means he calls down if a lot of crap post flop if we don't have any other information.

This could be Tx also, or some amibious draws that made a pair. If he was calling you down on flop and turn with a T, there is no reason why he would fold the river to a 2/3PS value bet. Even if he can read boards he's not likely to be worried about you having a straight.
I agree that most of his range beats you on the river, but giving yourself 20% equity is too pessimistic imo.