What, if anything, should be done to level a grand piano when the floor slopes down by 0.6" (1.5 cm) from treble to bass at the keyboard and by 0.8" (2.0 cm) from front to back?

If you want perfect level, just get some castor cups (if you don't have them already) and put some spacer wood under them to make up the difference. The 3 legs will always stabilize the structure without rocking.

Thanks, Emmery, that's the plan. Someone once said an action works best when the piano is perfectly level but, reading between your lines, reasonably level is OK.

Archimedes had the answer: Get a lever long enough and a place to stand, and you can move the world.