Are you building a bomb shelter or just a little 3 foot extension off the back of your shed here? If it were me,I'd just throw the block under the center for support,there isn't much weight there and the plywood sheeting will keep everything buttoned together.If for some reason code enforcement comes by or does one of their aerial searches ( and yes,they do fly around with helicopters here taking pictures) you won't have a major project taking down something you don't have a permit for.If your looking for nearly indestructable,I would have just poured a cement foundation and used cinder blocks instead.Home Depot and Lowes sell hurricane anchors that look like big spiral twist posts that screw into the ground and have a round eyelet on the top that allows a bolt to pass thru and into the side of the wall.Here's another option,just buy a used,travel trailer that's been gutted and use it for storage,or buy a used shipping container but these aren't easily moved.Years back I had a gutted airstream that worked great as a storage shed and shop to work in.