The rook endgame looks pretty drawish to me. If 51...Kf7 52a7 kg7 53 Rb8 rxp 54rb5 kg6 55rb6ch. Umm maybe something like 54... ra3 55kg2 h4 56Rh5, but can't see black winning that either.
Black's best move?

Your opening play looks really suspicious. Didn't see point of 6...e6, and 7...Nc6 looked ugly. Maybe 7...Bc6 was more natural. Recently had a look at game Sajtar v Smyslov '46, which is in Smyslov's best games book. Think this one would give an idea on how to play the opening better. Think black is in trouble after 15d5.
I managed to get into a position which created a draw in this game, however, I wonder if black had any chance of winning at all...should I have played 51...kf7 or was the move I actually played the best move?