Chris-

The ULD values you're referring to are per container.  You can load up to 8(?) containers in the ULD so that would be about 72,000 lbs on the upper deck, if my memory of max containers is correct.

I don't have the answer, but I do know someone who flies for FedEx and I think she flies the 757.  I will ask her, but she's not going to be available for a while.  I'll try to remember for when she's back home.