Looks like you are running Caster battery trays.
If so i have had excellent experience running the Zippy 2650 line of cells with those trays. The cells are the same dimensions, which fit perfectly and the height depends on which pack you buy. I use the 4s in parallel but have had the 5s fit fine, as I assume the 6s would too.
http://www.hobbycity.com/hobbyking/s...35&search=2650