You will have to make sure you are on your PC and checking in right when check-in opens. You are flying a long haul and those upgrades to BC will be harder to get as more people will be wanting them for the longer flight. If it were me I would just get two coach seats.

I am also a person of size meeting almost all of the same measurements as the OP, I have never needed a seat belt extender on AirTran. I am always flying with my wife and usually take the left hand side two seats on the 717 so being of size isn't an issue for us as the only people uncomfortable are the both of us.