I think no kart should carry more than 50 pounds. More than that requires some pretty creative ways to double stack lead blocks through 4″ long bolts. creating big levers on fiberglass seats.
I mount 2 lead bricks 10 pounds each to the back of the seat with required two bolts another brick to left side. a huge custom poured chunk under crotch of seat so that gets us to 40 pounds. and then squeeze little pucks were space allows. size 1 seat does not offer lots of area.