These are level, but they'll leave a 2-inch gap between the ground and frame.
Can't you lower the blocks another couple of inches?
If you can this should do the trick.
Leaving it open to the elements will 'yes' create ventilation, but in other situations it will cause draughts and if these get a bit icy then you could damage the crops within.
You don't mention if you are in an exposed area but if you are the wind is just going to whistle through the gap and god only knows what damage can result from this.
A gap will also makes it easier for vermin to get in.
You could spread a little loose soil around the base to cover the gap.
In other words what I am saying is get rid of the gap as I feel this will benefit you in the long term!