I've been working on my rear end rust repairs for the last three months

In all that time the car has been supported on two jack stands, placed under each sill.
If you look at the rear sill ends, there's the two bolts each side that hold the subframe support plates. Just next to them should be the jacking plate; a solid lump of metal welded to the sill. This tells you that INSIDE the sill there's an extra leg of metal to add strength to the sills when jacking up; that's where the jackstands ought to be placed.
Use a length of timber though, to stop your sill lips collapsing.