In your original blueprint you were using your SpawnArea object, and getting its relative location and Box Extent variables to control where things are spawned.
You can do that still in your current version:
The relative area is the same as the BoxCenter1 variable in your current blueprint, so you can get that from your SpawnArea actor. You can do it similarly for the Extent, you could add a Box variable to your SpawnArea actor, and then get it, in your function.