I would say the same thickness thats already on it, thinner and it wont be strong enough and I think if you use thicker it will be too rigid at that point and cause a crack at some point.
I`ve done a few repairs and extensions and i know the rules are different, but any structural framework repairs cant jst be a cut and shut to be legal, they must have a reinforced overlap of about 150mm per side of the join. Where possible i tried to do it in one piece with 10 mm plug welds either side of the join. I made up a U section that slid inside the chassis, sometimes i had to shorten one end due to the chassis shape but it passed engineering for a 1000mm extension on a brand new land cruiser then it should pass (or even be overkill) for your car