From what ive read about the Nissan VK56DE engine, they've sleeved the block (to bring it back to 5 litres capacity from the 5.6 it is in the Patrol), and counterweighted the crank (because the engine is too light)
Havent heard anything on the Mercedes side, if the Stone Brothers / Erebus merger is anything to go by, they'll be there (im sure Nick might be able to enlighten us on how many car operation Erebus already has in the GT championship)