Regarding the incident with the son of Rebbi Yehoshua ben Levy who got a glimpse of the world to come and related to his father - according to the Geonim cited by Tosfos - the reversed roles of Shmuel and Rav Yehudah.
From the aforementioned it ensues that Rav Yehudah passed away before Rebbi Yehoshua ben Levy.
Furthermore, the Gemara states in tractate Kiddushin 72b that Rava was born on the day that Rav Yehudah passed away.
Although it is known that Rebbi Yehoshua ben Levy merited a long lifespan, nevertheless, since we find him interacting with the earlier generations, it would seem somewhat incongruous that he would still be in this world after Rava's birth.
Mordechai Schwimmer Brooklyn New York
1) It is true that Rebbi Yehoshua ben Levi was of an early period, but it should be pointed out that Rashi in Nidah 18b (DH Shemasesa) writes that he was an Amora. Therefore, it is less surprising to know that when Rebbi Yehoshua ben Levi was a very old man, Rava was a very small boy. This is easier to imagine if we bear in mind that they were both Amora'im, so there could be a slight overlap in their lifetimes.
2) We can appreciate the above Rashi a little better if we compare it with Rashi in Nidah 26a (DH Amar), who writes that Rebbi Chiya was considered both a Tana and an Amora. However, Rashi only says this about Rebbi Chiya, which implies that when Rashi wrote a few pages before that Rebbi Yehoshua ben Levi was an Amora, this means that he was only an Amora, not a Tana. We learn from this that even though Rebbi Yehoshua ben Levi was early, he was still later than Rebbi Chiya. This also makes the chronology slightly easier to understand, since we know that Rebbi Yehoshua ben Levi was well entrenched in the period of the Amora'im.
Kol Tuv,
Dovid Bloom