I would run a chain and binder around the top of the stand and platen, one sideways to pinch both tops together and one lengthwise one each platen. Do that before trying to load and leave them on until unloaded. Sounds like you will need 6 chains and binders. 3 to hold tops to stand and 3 more to hold it to truck. Clamps will work also, use them as well to prevent any small amount of shifting. Tight chains will damage the corners of the cast iron slightly, protect it with something if it matters.
Sounds like you are in good hands.
Try to take pics as you go, it should be very simple.