The 13500 is a gross combined weight rating (gcwr). More to do with braking/engine towing performance/drivetrain reliability/etc. The truck maker may choose any number he wants. This does not mean the trucks components will break if that number is exceeded as evident a simple gear ratio change say from a 3.7 to a 4.1 can change a vehicles gcwr number.
Helper springs....... just helps the truck rear suspension carry more weight which is more inline with payloads rather than a tow rating. Keep in mind the truck has to carry weight (payloads)......and pull its tow rating.
This is where anyone with a tow vehicle should weigh his vehicles front and rear axles separately. That way a rear axle or a wheel or tire or rear spring pack capacity won't be exceeded.
Just stay under the trucks GAWRS and You won't break a axle or blow a tire.