Maxdexter: It helps a great deal when you've actually RAISED the snake from when it was small- as you'll know each other pretty well by the time it's so much larger.
It's a whole different thing to just acquire a large snake- it won't likely trust you for quite some time, & you shouldn't fully trust it either until you chalk up a lot of time together. But your question sounds like you're planning to raise the snake-?
One thing that can make a snake suddenly clamp down on you is fear, or feeling like they're going to fall. It's not always predictable, & you'd be surprised how strong even a 6' snake is when it wants to be. Boas have more bulk, but they're not the only strong snakes around. My current 6'-7' adult rat snakes are very capable of "hand-cuffing" me (they're great climbers but you sure don't want them looped around your neck- even if they don't "mean it"), while the 7.5' boa (BCI) that I used to have never did put any squeezes on me- but you always have to consider that it's possible & stay "aware" when you handle large snakes.