If he were caught what words could Montag cry out in his last moments alive to make an impact and wake up the public?

That's a very good question. I think that if Montag were ever captured by the authorities, he could do a lot worse than to follow the example of the old woman in the story who chooses to burn to death along with her books rather than allow them to be seized. In her last moments on earth, she cries out

Play the man, Master Ridley; we shall this day light such a candle, by God’s grace, in England, as I trust shall never be put out.

This quotation comes from Hugh Latimer, a sixteenth-century Protestant bishop burned at the stake for heresy by the Catholic Queen Mary of England. Alongside him was another Protestant bishop, Nicholas Ridley, who met the same gruesome fate.
If Montag chose to follow this example, he would be showing not just defiance of the regime, but that he understood why such defiance was necessary in the first place. He would've shown, like the old lady, that he understood what was at stake in challenging the regime over its wholesale destruction of the cultural treasures of Western civilization.