Taking out the twist

I welded up a stand for my bench mill the other day out of 3" square tubing and when I got done, despite my best efforts, it's slightly tweaked. I can't complain too much I suppose, as the tweak is only about 1/8" or so on a four legged stand two foot deep, almost three high, and about a foot and a half wide. The stand was started with a two foot horizontal tube and verticals underneath to make legs. Made two of these upside down U shapes. I welded two more pieces between them to make it stand square. There's an upper crosspiece at the top near the back, flush with the top, and the other is between the front legs at just the right height to put my foot. Each side is good and square itself, as I was able to keep those parts straight just fine, but the two are twisted slightly relative to each other, causing it to rock just a bit. I got it as good as I could when welding up by laying in a bead here and there to keep it square. I tried a couple things to pull it in, including using a jack to try and stretch the closest corners away from each other, but it won't stay. This thing is really effin' heavy and stout, so a car jack wouldn't push enough to actually bend it to stay. I don't have any other tools that I could think of that would help pull or push this enough to do the job. For now I have it in use, with a plastic shim driven under the "short" leg which actually does make the top sit level and not rock. I'll be adding other brackets and stuff to it later, so was wondering if there are any ideas about how to impart a little bit of a twist to welded tee joints of square tubing. I suppose at worst I could just weld in a small shim to the bottom, but wanted to see if anyone had any other ideas.