If your 6'4" all you need is 2" headroom and 2" legroom to fit just as comfortable as someone 6'
john@microsoft did this:
He had the small body frame rails under the carpet that the seat attaches to shortened and the seat bolted lower. can be done using OEM seats and not losing the OEM padding
http://nsxprime.com/forums/showthread.php?t=94124
You can also remove & shave the padding in the headliner,as much as possible and have it reupholstered.
both of those will also give you 2"
Other is Side mounted Aftermarket seats will give about 2" as well
so a total of 4-5" headroom and 4" legroom can be found in the NSX therefore theoretically accommodating someone 6'8
shaving the padding in the oem seat will give you the 2" headroom but not really help with leg room
For legroom, the only solution for that is thin fixed bucket seats adjusted as straight up as possible with all of the combined mods including shaving the rear headliner padding, should give 4-5 inches for the legs too.
I just did ALL these mods for friend. He is 6'5" and now fits perfectly with room to spare. headliner, fixed bucket seats and shaved seat mount frames.
A smaller diameter racing steering wheel will allow for some additional knee room as well.