Ive never been involved in this exact project, but from experience, expect a lot of little "gotcha" moments along the way. There always is.
In theory, if you shorten the frame to the length of a 6ft bed, you could simply swap in a complete bed from another truck. I would NOT try to shorten the existing bed. In a hot rod application with big tubs and it doesn't really matter, its a little different. But for this, no. I think step one is find your a bed from a crew cab truck, make it fit on the frame. It shouldn't really be that bad, any shop that does offroad projects or quality bagged/show/lowrider trucks can handle this. Its leaps and bounds easier than a stock floor body drop.
From there, im not sure why you couldn't mount a normal gas tank either. In theory this truck would be exactly the same as a normal crew cab, from the back of the cab to the bumper. The only thing different is you the cab size.
If all that worked, then you could press on with a lift kit. As stated, I don't think it would be that intense. Especially a small lift.