I suggest leaving it in FMS mode, and normal Cruise profile, for maximum speed performance. The plane will go as fast as it can at whatever altitude, right up against the ‘barber pole’ overspeed indicator…
But Weight (fuel) has an impact on top speed too, that’s a long flight so you probably had lots of fuel on board. Try going up to 41000, and as you burn off fuel (and thus lose weight), move up to 43,000.
Also, make sure you turn off Anti-Ice if you don’t needit, because that steals bleed air from the engines. If the speed indicator on the PFD is brushing up against the barber pole.
On the PFD, look for Ground Speed - which will be your KIAS plus tailwind.