I'm not sure what's going on with the 8' boards that they gave you but if that was the material that I was given I'd use the 4 - 14' boards screwed together for 2 beams for the frame to sit on. Since you can't get 20' boards, I'd build the frame in two 14' x 10' sections and attach the sections together on top of the beams. You'd only need 6 footings with this design.
There many be better ways of doing it but this is how I imagine the first 10' section looking like.
