I have the CVT Mt Caylee which is pretty much the same size as the Shasta, I have it mounted off the back. A couple of things which I really got ticked off with CVT and their lack of info on their annex is:
A) The door which faces the vehicle is the same size as the rest of the doors, which is only like 30" wide. Your tailgate is much wider, it is possible to bow out the door around the tailgate and hatch, but then the hole tent moves up and no longer touches the ground; making the floor useless and leaves a 6"+/- gap from the bottom to the ground (Truck is not even that lifted). This design was completely idiotic, since this would really be the only function of this door, is to give you access to your truck.
B) Back to my complaining about the door, it unzips from the bottom to the top, so instead of being able to leave the bottom zipped up enough to fill the gap left from the floor to the bottom of the tailgate, that is now completely open to the bottom of the truck. Not a huge deal, but we camp in the winter and that would have been a big draft sealer.
C) If your tent does not come with the separate annex rail, buy it. Otherwise you will need to completely remove your cover to use that same channel to attach your annex. Feeding the annex/cover into the rail is a complete PITA, especially by yourself.
I would buy the bigger tent. The Caylee/Shasta works great for two people, but more space is always welcome and you might as well grab it on the front end. Just prepare yourself for the annex limitations up front. Even though I love the RTT, it seemed like the annex design was by someone who had no idea what it would be used for in the real world. I set it up in my driveway once, and I got so mad at the whole thing I have no used it since. Really made me salty about the whole thing to be honest. I have really been happy with CVT's customer service and quality. I give the tent high reviews, the annex is a big fail in my mind.