I own a XXX-NT and the only thing that i like better about a RC10GT is that the front end is more open and easier to work on, but for that you must sacrifice the double layer cahssis which is not worth it to me
What exactly does a double deck chassis do for you other than help the chassis hold a tweak and also stiffing the chassis. A very stiff chassis will make for a ill handling in offroad that is why most people with a losi are always chassing the rear end of the truck exiting a corner.
O.S. is full of crap I have been running a cvr head on my cv's for several years now It has the same bolt pattern and the head slips right into the sleeve. Of course you are going to have to check deck height but O.S. always has their deck height set to high anyways.
I have a RC10GT RTR I have had it since the begining of the summer and I love it. I have never had any problems and it can beat all the XXX-NT's at my LHS Track. I have all alluminum A-arms and have a TEAM ASSOCIATED FACTORY TUNED PIPE which is awsome. I run a TRINITY 5 cell reciever pack to operate my servos. Very good truck has a strong stock engine.