I once took a B747-400 up to FL430 when we were light, just to see what it was like - all within the limits. I would imagine mass was about 220 tonnes on a flight back from JFK to LHR.
I took a picture of the PFD and it showed (can't be bothered to post image):
Mmo/Vmo at 259kts
max maneuvring at 255 kts
IAS of 240 kts (M0.847 just after NAT exit over Ireland)/FL430
Min maneuvring at 231 kts
Min speed at 199 kts.
So quite a big margin. Have however seen much smaller margins at lower levels of probably about 10 kts between max/min maneuvring.