if you can, get four 4" x 4" by about 4 to 6 foot posts well covered with creosote or another preservative. Place those posts on the slabs so that the floor supports cross over them and then build the shed on the posts.
If you have to put the posts the full length of the shed (i did not have to by the way) then use a staggered line so that the posts are about 6 to 8 inches from the outside of the shed.
You will have to get the slabs close to being level but this way the shed will "sag" to fit and should not move around.
Good luck
Bill Door