Yorkshire I don't think your three class 777 can lift 351 when ULR due to MTOW problems. Check FABS for places like IAH, DFW etc. The available econ seats have to be reduced by up to 30 seats. There is not much weight available for any cargo once it goes over 11hrs and seats start being reduced a few hours after that. So your 39% goes to 55%. Remember this is the old configuration ULR 380, with Crew rest on the lower deck taking up seat space. The new lower Crew rest configuration has another 28 seats in Y class, so over 60% more.
The 777 out performs the 380 revenue wise on shorter sectors when it can carry max ZFW. The 380 will carry max ZFW to about 15hrs.