About the only way i can think of to correct the warpage would be to cut it out again and start over,but this time only weld the thing with a spot tack n move type of method,don't weld it a 1/2" or even a 1/4" at a time..just tack welds,...tack a spot,move to the complete opposite side of that tack and tack there..etc....about every 4 corners opposite from each other let the whole thing cool for a few minutes,then continue again...and NEVER put water or anything on it to cool it off,,this will make it warp even worse.
Clamp the whole thing down in a way before welding it up so that it doesn't have a chance to pull,then after done welding leave it clamped there untill cool to the touch.
It can be corrected with some heat,or dolly hammering, but that's complicated to do correctly with out making it even worse,go to a few metal shaping websites or body shop websites and ask the pros there,they can help you with trying to straighten it out as is without starting over,they've helped me out with quite a few issues welding in body replacement panels with minimum warpage.
Don't DREAM your life, LIVE your dreams
Never forget 9/11
"Welding is like a woman,Get 'er HOT and Penetrate"
Gotta LOVE a person who knows everything about NOTHING
The only Thing necessary for the Evil to win is a good man to do nothing....
"How far you go in life depends on your being tender with the young,compassionate with the aged,sympathetic with the striving,and tolerant with the weak and strong--because someday YOU will have been all of these"....George Washington Carver
Want to know what an Ironworker is and the job scope of a Journeyman?..click here...http://www.ironworkers.org/becoming/careers.aspx