Q. I am 6.3 feet height. Is Thunderbird 350 suitable for me?

Yes . It is good for you as per your height. But the problem is, more height more shoulder pain you get when you go on a trip of more than 100 km ride. It’s good for local commute only, not for long trip.

Even I'm 6'3 feet height. It isn't suitable. It can only be suitable if you change the seat or add cushion to its original seat.....

Undoubtedly comfortable for city riding but if you plan on taking your bike for long rides then you will have to modify the seats and handlebar