My plan for two side-by side 1000's is to prop them atop a manhole casting frame (like the attached picture, without the lid obviously)
View attachment 104325 , get them perfectly plumb, then spot weld them to the frame. The frame has a flange of about 36" OD and 26" ID, which helps spread the load on the floor. My frames have holes cast in the flange so it's easy to put in a few concrete anchors. I work in the town where all the castings are made, and spec them all the time for my job, so it was easy for me to get ahold of a couple brand new 'defects' from the sales rep. If you go to any sewer contractor or city public works dept they'll have used ones lying around you can get in exhange for an equal weight of beer.
I have a telehandler so I can lift them into place pretty easy with a choker. Will weld a tab between the two and run a couple struts to the back wall of the shed just to make sure they don't move.
Though I haven't installed them yet, I think the ability to rotate the end of the tank around in the frame as I level it will make it easy to get it level, as compared to welded on brackets which would require shimming to get it dead nuts on.
The 10,000# full weight per tank is bearing on a 500 square inch area, so about 20 psi. No big deal, even for my foam under a 5" floor. i like coal reaper's idea of the plastic sheet. I may throw a piece of 1/4" HDPE I have under just to keep the casting from staining the concrete, as raw cast iron will do. I'm not worried about heat loss as I'm on a heated floor anyway.