At 6 feet tall there won't be any bikes that are too tall for you. That CRF70 is way too small, you shouldn't be riding that anymore.
If you have no experience with a clutch then you are going to struggle for a while. You will struggle a LOT more with a two stroke. Two strokes are far less forgiving of being in the wrong gear or sloppy clutch work. Climbing hills and picking your way across a technical section might prove very frustrating on a two stroke, especially until you master the clutch.
My first recommendation is to change your attitude about wanting to go "extremely fast". Just about any bike will have the capability of getting you up to a speed that exceeds your skill. Most of the bikes you have referenced will get you there very quickly, which can be bad.
You might be very happy on one of those two stroke bikes once you learn to handle them. The concern is that they might kill you before you can learn.
That CRF250x you referenced would be a GREAT bike to learn on. That is the bike that my daughter uses to race with and does very well. It is dependable and easy to ride and will have plenty of power for a novice rider.
You probably won't like it for long. It has electric start, which is great for my daughter and for an old man like me but a young buck like you would rather save the weight and kick start it. Plus the guys might tease you if you show up with a "happy button".
If you insist on going straight to the two stroke race bike please take it easy until you have mastered the throttle and clutch.
Above all, make sure that you have proper riding gear. Do NOT get on one of those bikes without a proper helmet and boots. Gloves, and goggles are also very important. I highly recommend a chest protector and knee pads. Elbow pads will save a lot of skin.
Just out of curiosity, how will you be transporting this bike?
And what do your parents think of all this????
Rod