Cairo vs Cyrano...
Of course, 67...Kxa6 is not playable on account of 68.b7
Nor will 67...Ka8 do as after 68.b7ch Kb8 69.Rb6 and Black can't maintain the cut-off on the 3rd rank. If the rook departs the a-file, then 70.a7ch wins at once.
So the rook has to stay on the a-file, pretty much. Here's a sample line: 67...Ra2ch 68.Kc3 Ra1 69.Kb2 Ra4 70.Rc6 Ka7 71.Rc8 and wins.
So Black must play 67...Kb8! 68.a7ch Kb7. In this line it is much harder - maybe impossible - to demonstrate a win for White. This seems to be due to the commitment of the WR to protect the b-pawn.Attempta by White to change the guard founder upon the Black rook's activity in checking the King:
69.Kc2 Ra2ch 70.Kc3 Ra3ch 71.Kc4 Ra1 72.Kc5 Rc1ch
(note, by the way, that this rook is not quite so committed to the a-file, as White requires a tempo to occupy it with his rook, giving Black a move to contrive countermeasures.)
73.Rc4 Rh1 (say) 74.Ra4 Rh5ch 75.Kd6 Rh6ch 76.Ke7 Rh7ch 77.Kf6 Rh8!