Take a look here at the specs for all the different T-56's. You'll see that while some of them are rated for 450lb-ft, none of them are rated for more than 3968lbs.

The torque rating for given transmission will be for a given vehicle weight. Add the weight and you add stress to the components. Decrease weight and you decrease stress.

http://www.ttcautomotive.com/English/products/T-56.asp