Looks kinda scary, when you add the rocks, or even expanded clay, you've going to put alot of weight on those IBC's. Yes they were designed to hold the weight, but not with any span on the frame. You should at teh very minimum support the entire IBC frame. Ideally a couple 2x4's one horizontal, and one underneath position vertically should support the weight you plan on putting on them cinder blocks. you might also consider cementing some of the blocks together, so they won't topple.