I'm only 5'11" but have a short inseam (30") and tall torso (more ape shaped than human.....) I have settled for a fairly leaned back position and remounted the seat all the way in against the tunnel to get my helmet off of the cage pad above the drivers window. I don't think it was a hazard, but I hated rubbing the pad all the time. I'm still close, but don't touch anymore. This also puts my arms and feet at a more natural extension.

As people have said build the cage around your seating position...and definately try a few cars out before you commit to a seating postion. The futher back you get your weight the better the front-rear weight distribution will be (and I assume that is good...someone with more experience in RX-7 setups may tell you otherwise).