I think this is a little too dark of an area in the story for that line.
Really? I think it would be the perfect time. Like imagine after Nagato finishes telling his story, Naruto tells how he's had it rough too. Then Naruto tells pain his answer for peace (I imagine him sayin it in a "you need to understand your way was totally wrong" tone, like almost yelling at him). Then Naruto says his catch phrase at the very end of his speech.
I mean, really...after all the times he's used it in the past, don't you think this would be the ultimate time to use it? Like all the other things led up to this point for that line.
Enh...maybe you could take out the "BELIEVE IT!!!" part, since that is a bit childish, but still, i think it fits. Especially considering that its been said that someone from Akatsuki will join Naruto. I think his catch phrase will make Nagato believe that Naruto will back up his answer, not just come up with it. Then when Nagato dies (which i think he pretty much has to at this point...) Konan will join Naruto in their quest for peace.
Idk, i just think it'd add alot to this point in the story lol
