I'm still learning, and enjoying my 92 flex Harrow 300, I have a few other sticks with higher flex numbers that I've been experimenting with and was thinking of picking something up with more whip, more to learn the mechanics of how to handle it.
I'm 6'4 265 would I simply snap a 75 flex stick if I tried to use it?
It depends on your mechanics. Probably not unless you're Chris Pronger. If your mechanics are good, then you wont snap it right away, but it WILL have a shorter shelf life for you than a 100 flex if you keep putting all your body weight into it. Of course the flipside is that for the time it stays in tact you should be able to fire off some lazers if you have the proper mechanics.
Of course if youre a beginner and you havent got the right technique down, then it's really unpredictable what is going to happen with your sticks. Poor mechanics (and everyone has them when they first start so its nothing to feel bad about) usually lead to either not getting enough weight on the shot, which means that the stick wont be under any stress, but youre not going to get off a good shot either, getting off some shots just right, and then loading your weight at the wrong place/wrong time which will snap a 75 flex or a 110 flex.
Once you get it down pat, the flex will become like a trade off for you. A lower flex will give you harder shots (to a point, you can have a flex too low and your stick will be equivalent to a wet noodle) but you will put a lot of stress on the stick and it wont have a shelf life of too long. A stiffer flex will last you almost forever unless someone chops it or something, but you will likely not be able to get your full power on your shots. In the end it comes down to what you like.
NHL players all get insane amounts of strength on their shots, and some opt to use flexes as high as 115 or 120, such as Chris Pronger, or Alex Kovalev, while others opt to use flexes as low as 60 or 70 like Alex Ovechkin or Phil Kessel. It all comes down to what you feel most comfortable with. For me anything between 80-90 gives me the combination of power that I like without putting too much stress on the stick from whipping it too much.