FL340 with a belly full of fuel is too high for a B747. FACT.
Transatlantic flights normally have an initial climb to the 270-300 block, then have another 2/3 climbs as they go. The aircrafts engines need the thicker air to propel it at heavier weights, they also don’t hit M0.85 when heavy, sticking to around .8, again until they’re lighter and more able.
Your GS determines your ETE. SO if you are going 350 from strong headwinds or no winds you are just gaining time. It should go down once you pick up the strong tailwinds
These two tutorials should get you started. Keep in mind, as you burn fuel your aircraft load will get lighter or decrease. Meaning less fuel consumption at a lower weight and higher altitude later on the in the flight. It’ll appear that your fuel flight time will remain the same or increase as the load decreases.