
I had my half stripped shell on bricks; half a dozen bricks stacked 2 high, with a thick plank of wood strectching across them, this was all under the front floor pan, didn't move an inch, and this was over the winter with rain and powerful winds.
Personally I think this method was as safe as sticking that plank of wood across axle stands, however I'd never use a couple of bricks under the engine or subframe.