You might use grade 70 transport chain. It is not as strong as chain rated for overhead lifting, but it is better than grades 30 & 40, and redily available.
Perhaps You could use Allans other idea using the jack and chain to srtaighten the square tube, rigging the chain directly to the tube, not to the plate. This might require welding some temporary cleats to the square tube, if that is acceptable [might violate a code].
Hello Pat, not sure of your current overall shop situation, but don't you possibly have some remnant sections of beam, channel, tube, or other structural shapes? If so, pretty much anything with some depth/thickness to it could provide that little bit of extra rigidity to help with limiting the flex. Even a mirror image of this set-up could be used underneath the table as you were employing the system on top to help with flexing. Hope this makes some sense. One additional note to go along with this sort of thing: as you are applying the force be very careful about where you position your body and also pay close attention to where others are relative to your set-up. The angles of the chain greatly reduce it's ability to stand-up to a lot of force and chains, cables, straps, and any other materials that are subjected to pressures will store energy that can release if it's stressed too greatly and can cause considerable damage to people and property. Glad to hear that this was of benefit to you, if you have "tricks" to share please include them on the forum here. Best regards, Allan