6'4'' and Sportspack seats in a 90, no problems.

 

I'm two inches taller than my son-in-law but he finds the XJS too short and his head rubs. The difference is the way he adjusts the seat back. He prefers to sit bolt upright with the steering wheel in his chest while I tend to lean the seat back and get further from the wheel.

If you are ambitious and a little handy with sheetmetal, the front cross brace and the two rear seat mounting pedestals could be shortened a good bit.