[CAVEMANN]

Metal pulls from heating and cooling, welding will tend to pull the metal, you can clamp it rigidly like uniballer said or you can continue as you are doing. I think uniballer's suggestion is probably the most logical choice, 2 on each side to hold it in place, that allows you to tack both sides, you will need to distribute weld on each side alternately to help with warpage. do you even need to weld both sides, do you even need to fully weld one side? How about skip welds on each side?