Villian is 38/31/0 over 32 hands. No showdown stats yet as we haven't played many hands.

I think preflop, flop and turn are pretty standard. My question is about the river, does anybody not like this bluff?

I think its unlikely he has a FH or a straight given his line + my blockers. A backdoor flush is possible but not likely, especially given my 5s and the fact that the Ks is on the board so 2 of his 3 side cards with his Kxxx would have to be spades. That leaves missed heart flush draws which will almost definitely fold, and AK. Do people fold AK on this river? I doubt I'm getting to the river with unimproved AA like this and unless I have a flush I'm probably just checking behind. My value betting range i guess is KK and most flushes. Given preflop I think villian can discount 55, 66 and hands with a 2 in them. 

Given my narrow value range how often can bluff in this spot? I don't have any hearts which adds to my case for bluffing. Against a villian who isn't paying attention I can bluff more because they will look at their hand and go "hmm, only one pair, fold" often. But against someone who knows my value range is KK and flushes only how often can i bluff and which hands should I do it with to make him indifferent to calling?