Hi ,
It sounds to me like what would be most useful to you is savegame objects. Using savegame object blueprints, you can pass information between levels without losing the data during loading/unloading. You can find that here:
https://docs.unrealengine.com/latest/INT/Gameplay/SaveGame/Blueprints/index.html
If all of the information is going to be streamed in, you should be able to access any info streamed once it opens, if you have information that is being unloaded at the same time, you should be able to use the save state to pass any values/information you need to keep track of into the next level, then when that level is streamed back in you can simply pass that information back.
If you plan on keeping the persistent level throughout and simply loading and unloading streamed content, creating a Game Instance blueprint and saving the information in there should serve the same general purpose, it depends on what specifically you are looking for.