Yes there has been a lot of good advice given so far. I am a mechanical engineer and if I designed a treadmill to handle 400 pounds of dead weight, I would probably put a manufacturer suggestion of max weight at 300 pounds.
If you were able to get through to the manufacturer or reseller, they would just tell you to observe the original manufacturer recommendations; they are not going to suggest that you exceed those (to protect themselves).
The weigh limit is one thing but the motor is another. A cheaper treadmill (under $1000) is not going to have a powerful motor. If you are using your treadmill and the thing stops dead, and switching it on and off does not restart it, then the motor has probably tripped out on a thermal overload. Let it cool down and you can restart it later, if you're lucky. Try to walk/run along with the motor/belt speed and don't let it struggle to move against you.
(If this sounds like the voice of experience, it is).