Number 8. if you're that big, then you should be a prop, but if you're that fast then you should be at the back of the scrum, killing the scrumhalf and pushing the scrum forward.
Your coach will tell you. Generally though, the linebacker body shape will be best suited to playing somewhere in the loose forwards. You COULD find yourself in the centres if you are fast enough though.
Ask your coach, that's what he's there for.
Flanker, if your decently fast and that big, flanker is good, you are a forward that is first out of the scrum so you are the one to make the quick tackles right away.
you're best bet will be a prop, but prop is a really important position.