13 hours...is that all? The trip to India involves an 8-10 hour trip to Europe, then another 8-10 hour hour trip to India. Pretty rough. Since the flights leave the East coast in the early evening, and don't arrive in India until early morning two days later, I end up missing at least two nights of sleep, which is worse than the sardine treatment in economy class. I am also 6'4", and have made this trip numerous times.
I have only gotten a bulkhead seat one time, and it did help a lot. I also got upgraded to Business Class on one flight and that was great. For some airlines you can check their seat configurations online and try to request those specific seats (some rows have more room even though they are not an exit row). If you are lucky the seat next to you will be empty. On several flights I have gotten my own row of 3-4 seats which means I can strech out and sleep. Otherwise I do try for a window seat, because I can lean against it and get a little sleep. Take an interesting book to read--that helps the most to make time pass. Get up and strech and walk around whenever possible. Some people say you should take a couple of aspirin to prevent blood clots in your legs. Also try doing some leg and foot exercises in your seat, and take off your shoes. This will help prevent deadly economy class blood clots.
GunLvr