If you run the tank the other way, is it different? my guess is you are running it the same direction as the driveway? garages slope to shed water. this slope is often about 1/4 inch per foot, which would be be an inch and 3/4 or so over you 7 foot tank...
If you must run the tank this way, one option is to frame an area the size of the stand & level the floor as suggested earlier. the bond to the floor needs to be considered.
if you run the tank the other way, the the slope will be less than 1/2 inch & shims under each front vertical support should work well. You may want to cut your own shims or have them made for you to the correct angle, as most wood shims you buy are too short (like a foot).... You could attach the shims to the base or leave them 'floating'.
If you leave the stand as is, and plan on using shims to level the stand, be sure to shim each of the 3 sets of vertical posts (the low ones) and use something to take up the slack between the tank & the stand; usually some sort of foam board/insulation. I am not sure the thickness you will need as I currently have nothing over a 55 & use 1/4 inch foam core to true the tank to the stand.
hope this helps.