Just to get BM, Naruto had to become friends with Kurama. Once they became friends, Genjutsu became virtually useless, so with his overwhelming power and the fact that Kurama could get him out of genjutsu like Hachibi did for Bee, Naruto would beat Itachi. As for Nagato, well BM would beat Nagato. I doubt he could fully absorb an TBB, and even if he could, it would take a minute to do it. So, Naruto could launch a TBB then attack while Nagato was busy absorbing the TBB. Even if Itachi tried to interfere, Naruto could make a clone to keep Itachi busy. There's no way the two of the could take out BM Naruto with his super speed, overwhelming physical power, and Kurama protecting him from genjutsu.
Itachi and Nagato are 2 of my absolute favorite characters, so i'm not playing favorites here. this is what i truly think.
Even Chibaku Tensei would be ineffective; for one, one of Naruto/Kurama's giant TBB's could destroy it. and two, even if Naruto didn't think of destroying it (cuz Itachi's the one who thought of thought) he could bust out of it in full 9-tails mode like the Kyuubi did before. Or, he could simply have used his speed to get away and come back when Nagato ended CT, i just don't think they'd beat BM Naruto. after all, BM Naruto means you are fighting both Naruto and Kurama at the same time and they have each other's backs.
that's my opinion. if anyone disagrees, that's fine, just please be respectful about it