I think it also depends on the kind of rack. I do mine length way as well. I also put 6 tanks per shelf back to back
You can see that the board actually sits on the top of the wire in my case, so for extra strength I extend it out over the arms. (Ignore the mess.)
The weak spot doing it this way is that the most weight is in the middle of the board. So, 600+ lbs does cause it to sag a little, but the wire is a nice reinforcement. If I were to put the tanks width ways, there would probably be less sag since the arms would take most of the weight directly. As it is, the arms take only the front weight . However, I accept this since I like to see the entire view for selective breeding. So, I adjust my water level as needed per tank due to any slope.
I also drill some holes in the board and zip tie the wire to it. Since the wire (is supposed to) sit in L channels, the wire helps to keep the boards from moving.