I'm 6'4" and I don't have any problems. I have the seat all the way back and all the way down. I also lean the seat back quite a bit, but I have been driving like that for over 20 years. My head will only hit the liner when I lean my head back on the headrest with the lean, and that is because it sits where the roof beggins it's sharp down slope. I do want to track my car, so I will have to try it with a helmet.

Having just sold an '08 vette, I'll tell you the Z's seats are FAR superior, both from a construction and comfort/sportiness factor. When you lower the Vette seats you'll literally feel like you're sitting on the adjuster motor the padding is so sparse. The one universal complaint about the Vette is the seats...
(It's also quite a different car)...

If you are saying you are 6' 3/4" then that is a far cry from 6' 3".

No, 6 feet 3 to 4 inches tall, I have not been "official" measured in a few years and the last time it was 6 feet 3 inches and I have probably grown just a bit more.

I understand the reasons for sure...that plagued the Ford Mustang up until the new S197 body that came out in 2005. When they redesigned the car they actually added several inches to the car interior design in order to accomodate folks up to 6'5" as people 6' 3" and greater didn't have any head / leg clearance in any of the prior models.

I am not so sure the Z can make such a similar redesign move as the Mustang given that making the Z longer would be entirely the wrong design move to make.

In some reviews I read that the z is not designed for people with heft.
6'3 to 6'4 is max. But if u have extra meat. Your backside may make u sit
Higher. Also your center of gravity may not allow u to sit properly.
Sometimes in these type of cars. You gotta buy an aftermarket seat
To get your body in good. Recaro has great sports seats for heavier set guys.
Don't know if this is your situation. But if it is? It might be better to buy a
$900 seat than an extra $10k for a different car. Maybe?

Don't actually own one (yet), but I've tested several times, and at 6'6" I've got headroom to spare, and no problem with legroom either.

As everyone says, it it depends on how you are built. I am 6ft but more torso than legs. I drive an M35 because I found a G35 a little short height-wise (and I LOVE the ventilated seats).
I rented a vette in Florida last September. Luckily it was a convertible so I would just look over the windshield at red lights. With the top up, I would have to lean down to see under the visors!!
If you want HUGE headroom, try a Boxster. Also the MB SLK55 AMG is pretty good too.
My M35 is due for a service soon so I will get to try the 370Z.

Good luck!