So far we are very happy with the decision to get a shell and build it out. That said, we've had little chance to try it out since the mods were completed. We are actually headed out tomorrow to give it a test run with the latest addition - homemade thermal pack (write-up on that to follow very soon also).
We did actually buy a new thermostat, as the one that came with it would only go down to 45 deg.(I think). The new one is bigger and needed to be moved to accommodate the rail extension you noticed. That extension is backing for the plywood seatback that leans against it when you're sitting, facing the table. When you're sitting longitudinally the seatback leans against the bed rail. Relocating the thermostat was pretty easy. On the advice of our dealer, we ordered our shell with the interior skin screwed on wherever possible to allow us to unscrew it and peek behind to see where the aluminum framing is. That has been helpful more than once, along with these pictures of typical framing:
http://www.wanderthewest.com/forum/topic/7050-stan-question-about-fwc-frame-weldment/
We are happy with the options we ordered. Some we haven't used yet (like the rear wall steps) but think we should have for the future. Everybody's needs are different - we wanted it pretty basic, i.e. no built-in stove, no built-in water and for now, just a cooler. We have a one burner Coleman we can use inside to make coffee but, aside from that, we want to do all of our cooking outside if possible. We removed the back seats in our Dbl Cab Tundra (a ton of extra room, even if you just remove one) and keep the cooler in there.
If I remember to take my laptop, I may get the build written up while we're on this little jaunt. If not, soon after I return.
I'm happy to answer questions, Jim. This forum has been such a help to us along the way I'm pleased to be able to give something back.