I am 6'3" and weigh 230 lbs.
I almost didn't buy a Viper because of the lack of head room. But, I had a Northwind Enterprises seat lowering kit installed and that made the difference.
With the Northwind kit, I can sit completely upright with a helmet on and my head does not touch the ceiling. So, if you are 6'5"... you should be OK. But, you might not be able to wear a helmet.
If that is the case, you could purchase some Sparco racing seats and have a special bracket made. That is what I am doing in my Supra. Beacause I cannot sit upright in the car with a helmet on.