If you can identify where it's out of square, it's going to be better to fix it. A 45 degree diagonal cut with an abrasive saw blade should shortren the long side enough to correct the misalignment, and you can pull the gap together and tack weld it. Check everything out for proper dimensions before doing the final full welds.
I like to do my frame buildup on a totally flat, level floor, using jackscrew type stands to align all the pieces before doing any welding. If the floor happens to have built-in chain anchors, so much the better. With a little tension on a few chains, a frame can be tweaked into any shape the builder desires. On the round track cars, we will sometimes build in a little twist or out-of-square on purpose, but all we're interested in is "Go fast- - - -turn left!"
Jerry
