The best thing you can do is bolt them down to a spare short-block.
Do not set them on concrete.
I have a set of heads stored for a couple of dacades now within a large home-made wooden shelving unit, made mostly of scrap 2x4 lumber with 1x6 as the shelving. There are some intermadiate 2x4 supports under the 1x6's, but it's not perfect.
Anyway, the milled surfaces look like they were just done yesterday, but I don't know if any warping has occurred.
If you have to store them without a short block, I suggest sitting them on perfectly flat 1 x lumber with rocker covers installed on top of them or some kind of cover over them to keep dust and insects out.