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since nobody else has mentioned it ... bigger tires/rims and brakes.
sounds like you are planning on towing heavy ? if so, you may want to pay extra attention to the word Conanski mentioned ... "legally". it's all fine and dandy until there's a collision; that's when the lawyers start circling the carcass.
Not worried about a great increase in capacity: I just have a 5200lb WWII Dodge to tow, and I anticipate around a 1000lb trailer(est), and a 6800lb towing capacity, according to the literature... It is not a regular towing issue- just an occasional one, but I always try to improve things. That and I want to eventually add 4WD...
As to "legality" unless you are attempting to grossly overdo things, it is in the eye of the beholder in this instance... I am not...