i would try get a styrofoam to even it out. just to be on the safe side. rather than to regret it when things go wrong. it would be hard since garage pavement are usually a few angle down to avoid water pooling.
OMFG! Yeah man!
I would shim it. I know its heavy , but couldnt you pound the shims in with a hammer?
More pics! Its kinda hard to actually see how big that tank is without a reference. More pics!