I run a table over mine, and my passenger rides on top..
Strapping them down to a floor can be a good option.
There is a strong possibility that you will need that space for something else too, so keep in mind other ways to move weight forward.
You also don't HAVE to take that much water, like you do on trips like Cataract.
This winter we took one-two water jugs per boat , and a Katadyn pump style filter, filled jugs at phantom, and only pumped a few times.
Have an awesome trip down there!