[Kalun D]

Your chassis will have less warp if you anchor your jig to the floor. What happens with a light jig like you have is it has no torsional rigidity unless you anchor it. You can get the chassis all perfectly straight while it's tacked but with the heat of the full final welds it will twist.

If you look at some of the pro builders they have a monolithic jig table that can't warp.

Also realize that you will get some warpage no matter how ridid the clamp down is during welding. It will be "sprung" at least a little no matter what, so if you anticipate this you're better off. If it's an unacceptable amount you can jack it straight if you have some anchors in your slab. Minor tweaking will not hurt mild steel.

One thing you should do is have all the tubes already final welded before you weld any of the suspension brackets. That way you can place the brackets for optimum alignment no matter where the frame rails end up.

For example it's acceptable to have up to 1/4" warpage in the main chassis rails but it's not acceptable to have that same 1/4" innacuracy at the suspension mounts.

You can also minimize chassis warpage by welding in a strategic order (I call it the diagonal opposite method) but it sounds like you already have some racing building experience so you may already know this.