This job may well be better suited to RailClone
which is designed to create array based models just like this, and also has the instancing power of Forest Pack.
However you may be able to do something similar in Forest pack using Tree Editor > Creation Tools > Along a Path
mode which let's you distribute objects along a spline. To use it you'd have to create a spline for each floor and set the Spacing value to match the width of your element (800cm).
If you create a separate FP object for each floor you can control and randomise variations and in Tree Editor
mode you can easily go in and make manual adjustments if necessary.
I've attached an example setup which I hope can be useful but please let me know if you have further questions.