Yes you should rep the ace on the river there and act as if you do in fact have the ace it woudnt be hard if its a good player your facing for him to fold 66 considering the ace beats him and the other 2 cards on the flop beat him as well the 7 and the J so if you had made a reasonable bet on the river, he would have lied down the pocket 6's. I know what you mean though sometimes its just more profitable to check the river and save your self some money instead of betting knowing the the player might call you down with a low pp.