I'm think of getting a battery that will fit in my buffer tube but not put too much stress on my gearbox, and I've heard that a 7.4v LiPo is about the same as running a 9.6 NiMH.
I have the 1300's and they fit fine. But if you don't want to mash up your wires you probably need the stock extended at least one click. And you can ditch that spacer thing with three circles.
I use 7.4v lipos in all my guns and I fit deans plugs too as tamiya plugs are just outdated and really don't help the battery to get the power through.