I didn't see any mention of this, but you will most always cut and shape both your patch piece and your cab like someone said above, you only use as much as you need.
As for warping, I had a #(*& of a problem and the solution is generally to watch out for pre-stressed areas before you weld. after the metal heats up it goes nuts trying to relieve the stresses. Or if you let too big of an area heat up. It will warp from the expanding metal trying to go somewhere.
I've used heat sink putty, copper backing, drywall spackle, etc., and the system that's most successful for me is just to blast it with compressed air (both sides if possible) after you make a tack bead (emphasis on the tack).
