Realistically you want it as accurate as possible,but unless you are experienced welder you will always get some distortion.I think if you are within 2-3mm overall thats pretty good.You will be able to conceal this easily whether you make the panels from fibreglass or aluminium as there is excess material in the fibreglass to trim and if you make the panels from aluminium you can add the extra.I do recall reading somewhere that someone measured a chassis from a certain manufacturer and one side was longer than the other by 10mm
