All great replies, so I only have one thing to add to what has been said thus far.
Find and purchase a copy of John P. Stewart's "Flame Straightening Technology for Welders" for your personal library.
This book also gives desgn tips and examples of fixturing to prevent warpage, techniques to use while welding to prevent or minimize warpage and distortion, and also examples of how to straighten welded items after they have been pulled out of shape by the heat from welding/cutting, etc.
The ISBN# in my copy is 0-9692845-1-9