Difficult one to answer.  In theory, I would say that up to the height of a normal shipping container should be okay.  However, it all depends upon the weight of the container units, the type of road transport being used (and whether it can handle the weight legally), and how you intend to secure them during transportation.  Without that sort of info I couldn't even begin to hazard a guess...

On the face of it, this sounds reasonable to me.

The main things to consider are a) the height and safety of the load and b) the capacity of the vehicle - including axle weight limits.  If you're absolutely sure that the load is secure and can't move or slide (and it's not so high as to risk hitting bridges !), I would then check with your Transport Manager / "O" Licence holder to make sure that the vehicle is suitable for the load.

I'm assuming of course that you have the equipment/capability to load them in this manner in the first place.