6-belt swapper
Gap between inserters: 3 5-belt swapper
Gap between inserters: 2
Requires 1 extra tile off the belt. Underground swapper
Gap between inserters: 2
Blocks all previous items on the near side. Pair of balancers
Gap between inserters: 2
You need two of these, one exactly 50% of the way down your production line, and one at exactly 75%.
![Image](https://forums.factorio.com/images/ext/b6b6aaedc632c5a8008be0448fb9f2c2.jpg)