I'm 6' 4" too and it was far too cramped with my (manual) seat all the way back. As a temporary measure, I took off the seat, removed the riser brackets and re-installed the seat with bolts run up from underneath the car--to the cages nuts on the seat assembly--on the front only. (The back of the seat sits on a piece of plywood in the back footwell to avoid drilling new holes.)
This is not ideal. There's nothing holding the seat down in the back, the back of the driver's seat is right up against the back seat and there's no way to adjust the seat. Also, the seat is now a little too low!
My plan is to remove the sliders from the risers (They are riveted together.), and essentially mount the seat and sliders without the risers. Luckily, I have manual seats, so there's no motors and electronics to work around.
Hope this actually makes sense. I'll post some photos when I finish this project.
~Steve