You can look at your owners manual for your year and model to get precise cargo capacities, but as a guideline, they assume each belted seating position can hold a "standard" person at 150 lbs. So if you had the 3rd row up, with all three seats filled, it would be rated to carry 450 lbs worth of passengers. Folding the seat down and putting 400 lbs of cargo in there should be easily within the limits. That's especially true if your second row isn't filled to capacity.
The point about securing it is a valid one, of course. I'd be tempted to leave the second row seats in place even if nobody's going to sit there, because they will act as somewhat of a barrier to keep your bulky cargo behind you in case you have to stop suddenly or if you crash.