First 359 chapter is in one piece world with main char using his very own zanpaku thousand magic (yeah it can basically using all other zanpaku ability) along one piece world of haki and body strengthening till he conquer the whole world and become overload in one piece world. (Don't worry luffy still get his one piece using identity of adventurer instead of pirate)
Then the next 207 chapter, main char goes naruto world and repeat strengthening using chakra, natural chakra, path of six and merge those power with his shinigami and repeat conquer naruto world to become overload.
At last 143 chapter of him going to bleach world and conquer the world and become overload.
You see the repeating part yet ? He keep going from world to world absorb different power system and conquer everything like some sort of ego to fulfill. The chapter is getting shorter and shorter because the author running out of thing to write. After the first world the next 2 world very quickly become boring because of repeating theme. If only the author end the whole thing at first world it will be a great 4/5 star story but if u add in the next 2 world I don't even know it is deserve 3/5 star.
By the way https://www. novelupdates. com/series/the-strongest-hokage/ is author earlier story with similar setting but only with naruto and one piece world. And the whole novel stink of repackaging so try not to read both novel together.