X2 on getting weighed, I'd weigh both the truck by itself, then connect and see where the pin weight is to make sure it's not too light.
That being said, on another fifthwheel I changed from the stock pinbox to the Morryde and saw a noticeable difference. A year or so later I changed fifthwheels to an Arctic Fox which came with a Morryde pin box on it, the distance to the wheels on the fifth wheel are about 3' shorter which also improved things. About 6 months ago, I went from a Extended Cab 2500 Short Box to a Crew Cab 3500 long box which improved it some more. I rarely get any chucking on a road that isn't all beat up.