It would probably require a whole lot of 2x4 supports spaced closely below the top frame in order to limit sag...which would limit the size of the opening underneath to get to your sump. You're dealing with a tank that is going to have a weight around 3,000lbs. For a 300g tank, I would look into a steel frame to put my mind at ease.
The other thing to consider for this size tank is where you are planning to place it. If it is directly on a floor with a concrete slab underneath, you will be fine. But, if the tank is going to be on a floor that is raised with a crawl space or basement beneath it, you will require additional support under the floor/house.