Choosing the correct flex in a ski boot boils down to the following: your ankle mobility, your weight, and what you have been used to skiing in the past. These are the 3 key ingredients to having a boot that works well for you. As far as your ankle mobility, we have no idea (that's one of the things a good boot-fitter will asses) but we do know your weight, which is not light. On paper, you would not be a good candidate for a 65 flex boot, but it is hard to tell without seeing how much you flex the boot.
Ankle flexibility tests are best done by someone who knows what they are looking for- lots of variables at play. But generally, if you stand up with a slight bend in your knees (without shoes on), keep your feet flat on the floor, how far forward can you flex at the ankle before your heels lift up? Can your knees get over your toes? Or do they stop short of your toes? Or do they continue past your toes?
Stopping short of your toes means you have a limited range of motion and you should generally be in stiffer boots (but a stiffness you can still flex). If your knees get to your toes but no further, then you cannot be in a boot that is too soft for you. If your knees go past your toes, then you can be in a soft or stiff boot- it doesn't really matter based on your flexibility. My knees can go past my toes and while I can ski a soft boot, I would personally much rather be in a responsive, stiffer, more efficient boot.
In terms of defining "how much flex is too much?", hop into your boots, buckle them up and flex forward. If you can flex the boot so much that the cuff comes in contact with instep buckle or the lower shell is bellowing out like crazy, then the boot is far too soft. If you can't even move the cuff and are not able to get your knees over your toes, then the boot is too stiff. An ideal amount of flex is one that allows you to get your knees over your toes but is not so soft that the lower shell bellows out or you cause the cuff to come into contact with the shell buckles.
If you need to stiffen your boot, your boot-fitter needs to drill holes through the cuff and shell and then essentially lock the cuff to the shell with screws & t-nuts. The cuff will no longer pivot at the hinge, but the plastic itself will still give. This will essentially boost the stiffness by roughly 15-20 points.