After all items connecting frame and body have been removed, one uses crowbars and shims to lift the body high enough to insert boards. First front-to-back beams are inserted between the frame and the body. Then crossbeams are inserted under these. You want to arrange these to distribute the weight of the body as evenly as possible. As you can see, for each corner the beam rests on a cinderblock stack while another tower of cinderblock and wood is made just inside the first stack. The inner stack is to support a bottle jack (I used a floor jack since it was what I had, this is not prefered, as they roll). The jack raises the beam, then more blocks are added to the first stack. Repeating this process for each of the corners eventually raises the body to a sufficient height to allow the chassis to come out from under the body. Remember that the chassis has to clear both the body and the wood that sits under it. The body now sits on four beams of sufficient magnitude.
Four cardboard tubes of sufficient height (Height of beam plus a couple feet for underground. These are planted in front or in back of the cinderblock stacks and filled with cement mix. The hole the tube sits in is also filled. Before the cement dries about 8 inches of 2X6 has four nails driven through it and is placed into the wet cement so that the nails are submerged in the cement while the 2X6 rests on the surface. After the cement dries, "tap" each corner of the crossbeams off its cinderblock tower onto the cement pillar. After all four are on the pillars, nail them to the 2X6 using double headed, or cement form nails. This is so you can remove them. In one corner I left the crossbeam on the cinderblock tower. You could do it for all of them, but I thought a solid pillar that goes into the ground is safer.
Your crossbeams are likely to bow slightly in the middle. I recommend adding a stack of cinderblocks in the middle of each crossbeam to reinforce it.
The only two problem I had with this arrangement were
First; the front-to-back beams only went back to just fore of the rear tire wells, because everything aft of that was about 1 foot further in than the rest of the body, so it was unsupported by the ft2bk beams. When one stood on the body in the rear door threshold, the front would begin to tip up a little. Sketch. Use ladders and avoid putting your own weight on the body. I suppose one could remedy this by screwing the body to the beams.
Two, lowering the body is difficult, landing it perfectly on the chassis by yourself is quite difficult. However, I managed to do it with only a couple of scrapes, and I had a broken hand at the time.