A few thoughts for what it's worth. There are lots of ways to do these things. I would build a continuous beam for the 30 foot span and I would let it rest on the top of the 6x6 rather that fasten beams into the sides. If you build the beam that is three layers of 2x10's, with two layers of 1/2" plywood laminated it will be the same width as the 6x6. Then you can add some angle braces on the face of the post and beam. This beam will probably end up in the 400 lb range but it will be very substantial.
As far as the loading, the 40 psf live load and 10psf dead load is pretty standard code for occupied spaces. However, if you're building this for your own storage use, you can design it for a lower load. I would guess it's really unlikely you will ever store 10 tons of stuff in this area. So don't get too carried away with meeting code exactly.
