You could go with a leveling kit, but then it might sit funny when towing your gooseneck... I'd probably still recommend it, though considering how you describe your 5er sitting while towing. Yes, you can get the offset (also called extended) pinbox for heavier trailers. Mine's got one and it's rated for 16k, although my 5er is only 12k. Oh and you can safely add an "add a leaf" to an existing spring pack with no problems. The only concern would be if you had overloaded your existing leafs and they no longer had any arch, especially since you are just breaking in that truck with only 91k miles on it!