Find out what is comfortable for you, I'll tell you since you're 6'4 you should probably use a more upright stance, not robotic or anything just more like a Lennox Lewis or if you like footwork use a style closer to Ali's.

Best punches/combos for taller fighters usually come off the jab so work HARD on the jab make it quick and accurate it doesn't have to be hard to be effective just quick and accurate. The right cross after the jab is what is known as the "1, 2" and having that combo down pat helps out alot.

Smaller fighters or fighters with better defense will try to slip your jab when this happens shoot the right cross or an uppercut to stop them from getting to the body or turn the jab over into a left hook and step to the side.

Hope that helps