G'Day...... Welding distortion should not occur if done properly and welded in a sequence. As Robin was saying MIG is one of the better options as has less heat input.
TIG is the ultimate best on thin plate but slow and costly.
Stick or SMAW (US), MMAW (UK) has a much higher input so the precautions for preventing distortion would be a lot more, but is dificult.
The weld is like a cast, as it solidifies it shrinks, so you weld in stitches of short lengths around the patch, never long runs. Also depending upon the curvature of the plate you may finish stitch welding too opposite sides instead of all four.
Gas is also good but it's a dying art.