One way to do it is to jack the whole thing up high, support it with something like concrete blocks (yeah, I know they can crumble, but I've done it even with boats to remove them from the trailer and never had a problem) then lower the frame away from the body and roll it away.
What I do is I put a 2 x 8 pad of wood on top of the blocks and turn each layer of blocks 90 degrees so that the bottom two are facing north and south and the next two are facing east and west, etc, etc.
If you do it on a solid floor, take your time, keep jacking it higher and higher in steps, eventually you will get it high enough. Just don't go getting inside the blocked up body and dancing around, and you will be ok. Then just reverse the process when the new frame is ready to go in there.
I am planning on doing exactly what I described above when my 46 parts car shows up so I can get the frame and running gear out from under the body. Having 2 or more floor jacks really helps too.
Don