what if? only 3/4? only? that's huge over 6 ft! look he originally said wood...metal shims may work...but a 3/4 run out means hes shimming up the whole tank...if we were talking a 1/4 inch in one corner ok but your talking about putting almost the entire weight of the tank on shims....so unless your talking about making steel tapered shims 6ft long x2 plus a 3/4 filler on low end, which i doubt will happen , that leads me to believe you mean shim up the end,,,then what? add more shims every 2 inches or let it ride like that? and thats not a hypothetical statement about the weld...if you shim that much you are changing the weight distribution of a ridged stand and you WILL drive higher loads than the original design...and im telling you from what i know if you start messing with the pounds per square inch of a welded assembly a weld may fail...if it was my tank i would pour a pad...simple easy and the right way to do it...everything else is half assed cobbled together junk
to the OP ...the statement about new concrete not sticking to old cured is true...id skip the barrier completely...