Hi Dave, and welcome to airforums.com! I'm 6'5" tall, so I know some
of where you are coming from.
If you look at the specs, you will see that some of the new ones have 6' 7.5" clearance, but the old ones, I can't say. I can say that our '74 model has enough head clearance where there is no ceiling light, or A/C grill. That levaes a few feet open to me to stretch, but not you, I'm afraid...
For me, the worst thumps on the noggin I've had are from the two bulkheads, one that separates the galley from the sleeping area, and the other between the sleeping area and the rear bathroom.
I had to wear a hat for the first month or so, to avoid the scrapes on my scalp!
I have to admit though, most of the time I'm in the AS I'm either seated (eating) or sleeping. So wear a hat, and you'll be fine!