If you could create a double seal so that when the cab was "open" it "broke the seal and closed it would reseal. I personally think that most people prefer conventional trucks for two reasons. One, the ease with which you can build directly (or with a seal) to the cab and two, aesthetics. I think most people are willing to give up the length and efficiency for the cool factor of a conventional cab. If you want to maximize the efficiency of the interior space a bus conversion, where you said you have experience, might be the way to go.
As to maximun length, that is determined more by what is allowed vis-a-vis overall length and what the law allows. Make the frame rails deep enough and thick enough and beef them up correctly and technically, the sky is the limit.
Tony
http://www.toolsetmotorhomes.com