Robert,
I'm 6'2" and couldn't drive my GTD when I bought it. (Incidentally, the one you're looking at can't possibly be 1965 - 1995?).
I had to fit a Gurney Bubble and remove the seat runners, bolting the seat to the floor.
The most complex surgery surrounded the impossible narrowness of the pedals as I could only operate them all at once! This required fitting a Tilton pedal box (which in turn required the heater/aircon evaporator to be turned through 90 degrees).
I still have to wear special driving shoes which have very snug welts and thin soles, but I can wear a helmet - as long as I engage it in the Gurney Bubble before closing the door!
The September 2009 copy of Fortyfication, the Enthusiasts Club magazine, carried an article on dropping the floor of a GTD, which in retrospect I think would be preferable to the Gurney Bubble.
That said, I've driven over 15,000 miles in the car and it's by far the most comfortable that I've ever owned.