is that a thermostat on the wall behind the 75? Not good. I would relocate the thermostat.
Also, I have the same tank and needed to shim the two front corners. After placing the shims I studied the stand supports dimensions and how they are constructed. A shingle composite shim will definitely allow you to level it but only a portion of the support will rest on the shim where the shim is at at thickest. Also the composite shims I got at Home Depot were not as wide as the support it was under.
So what I did was used four shims per corner. Each shim is a very flat triangle. So, knowing the max thickness I needed I trimmed each the to the necessary length and stacked two shims on top on each, opposing each other. This made a rectangle that the stand support will sit on completely. I repeated this and put the second set of stacked shims next to it. Now I had a mini platform of the necessary thickness that the entire corner support sits on. For me this platform is about 1/8 thick. I also let it stick out from the edges of the stand so downward pressure is not right on the edge of the shims.