I had the tranny forward chassis, I did assemble it but never ran it. I did not like the weight distribution, way too much to the the rear. I was also not impressed with the battery mounting, or available room for the battery. Nothing against flm, as I own and enjoy alot of their products, but this just fell short of my expectations. Monster trucks need the weight up front so they handle and steer well, plus it helps with wheelies if you are running a potent setup. And they fly alot better off jumps, as the weight helps to counteract the gyroscopic effect that large tires that like to balloon have.
I understand your desire to run the batteries inside the chassis, I wanted the same thing. In the end I sold the chassis at a loss and made my own tvps to accomplish my desires.
Here is the thread about that build:
http://www.rc-monster.com/forum/showthread.php?t=20077
I have not finished yet, but think it will turn out well. Battery placement is a bit high, but if you could stretch the chassis a bit more the battery tray could be dropped down quite a bit. I had planned to use 5s lipo for this, and length was a bit of an issue. Mine is setup for a battery that is around std stick pack length, 140-150mm?. I also wanted to use regular off the shelf parts, so I have a regular X length rear center driveshaft, and a lst1 wheel cvd as the front center driveshaft.
Not sure if you are into fabbing up anything, but this was the best option I could come up with! Much better info is in the thread I linked, my brain is a bit foggy from all of the muscle relaxants I took last night for my back!!!!