If you need to add 40# then you are likely on the shorter side and your center of gravity will be too low to begin with. Putting more weight down low will lower the C of G even further. The additional static weight from the 40#s will increase the rear tire load an additional ~12#, which will make it that much more difficult to lift and keep the inside rear hiked up in the corner. Moving the weight higher on the kart will help get the chassis to twist and lift the inside rear. So I would think the best place to put weight is on the seat.