you can start your post frame construction as i did when i built my 36x24 garage in 2005 -- dig holes--set posts--wrap treated rim boards inside and outside of posts--then fill dirt--then pour concrete flush to rim board. after cutting posts flush to rim boards, i used excess treated posts to "fill" gaps between posts and rim boards, which creates continuous nailing surface for perimeter stud walls. local building inspector approved everything. i filled up to 5 inches of rim board, for a 5 inch concrete slab. i used a number 6 wire mesh reinforcing web set at 2 1/2 inches (middle of slab), and secured the wire mesh to the rim boards by using 5/8 rebar "pins" in holes drilled at 2 1/2 inches down from top of rim board. this ensured the mesh stayed in the center of the slab at the perimeter.