warping. How to stop it?

[Bonzo]

Like you say, it is difficult to inspect a weld from a photo but I don't see anything that would have me unduly worried & judging by the distortion, you have managed to get some decent heat into the welds

Clamping to a plate may help a little but it will still spring when you release the clamps.

If you turn the welded plate over & find that the mill scale has blistered or the surface above the weld has wrinkled .... It is a good general indication that the weld on the other side has made good penetration & all is likely to be well

You only need to weld up the one side of the square tube ( Bracket side ) The opposite edge can be welded to the triangular plate that forms part of the top mount.