Can a Boeing 767-200 fly at 510 knots at a height of 400 metres?
The speed is secondary - what determines the physical limits of the Boeing 767 is Mach number and dynamic pressure.

510 kts at 400 m in standard atmospheric conditions equals Mach 0.775. This is well within the limits of the Boeing 767. But at 400 m it produces a dynamic pressure of 40,567 N/m², and that is too much. The maximum dive speed $v_D$ of the 767 is 420 kts.

This only means that flying at 510 kts is illegal, but it is still possible. If the airplane dove down to those 400m from enough altitude, it would entirely be capable to reach this speed, but would slow down once it stops diving.

There are several effects which can cause a catastrophic failure when flying too fast:


When the aircraft flies into a gust, the resulting load factor can overstress the structure.
When the pilot commands large control inputs, he will also overstress the structure.
Flutter might also cause structural damage.


The maximum Mach number of the 767 is 0.91 (0.05 above the maximum cruise Mach number, which is 0.86), and this corresponds to 523 kts in 11,000 m.  Thankfully, certification requirements demand a margin of 20% between the maximum speeds and flutter onset speeds, so flutter might be close, but is still tens of miles per hour away when diving to 510 kts. Remember, to experience flutter you need also to excite the motion first. Here is a good discussion of this topic.

In short, flying a Boeing 767 in 400 m at 510 kts is not recommended, but is entirely possible and most likely even safe when done in calm air and by a calm pilot. It only won't last long, because the engines will not produce enough thrust to maintain that speed. Flying this dive requires guts, but no special skills.