The best way to do this is to make your wall meshes modular. So for example, your wall mesh should not have length of e.g. 97, but make it full 100. This way you can use the UE Grid Snapping system to place your wall meshes and connect them easily.
Example of modular wall grid snapping: Custom Grid Snapping UE4 - YouTube
… But you can also use Vertex Snapping: Unreal Engine 4 (UE4.15) Tip : Vertex Snapping - YouTube