Do you really need to get all actors on screen or getting all actors of a specific type is enough?
Under Scene > Actors, you find the block [for each actor of type...]. This block iterates through all the actors of a specific type, in a scene. You can use it to set the location of you actors.
I remember your game has two types of blocks, horizontal and vertical. You can use the approach above twice, one for each type of actors, and, optionally, merge them all in a single List.
Do you know how to obtain the coordinates for a particular grid tile?