All depends where the center of gravity (COG) of the load is. I would expect that moving a tractor of that (small) size all the way to the front of an 18' trailer will do the trick, but you really should find the ~60/40 balance to get the best balance. Too much weight too far forward will squat the rear and pick up the front of your pickup. If you have an implement attached to the rear of the trailer it will change the COG, too. Load it in several places and drive around the block to get a feel for what works best.
Yes, more spring will help to stabilize big loads. Less bouncy-bouncy.
