I will start by saying I have very limited experience with these things, but personally I would highly recommend that you get an FG that you really like, and then just stick a crew cab on there. It would, I think, be a lot easier for you to accomplish.
Stretching the frame is not that big of a deal, if you do it right. It can cost you a bit, but it is not that hard. Best thing to do would be to cut it between the axles. Yeah sure, you have to get a longer shaft and such, replace cables and hoses, that is fine.
It is not "common place" to lengthen the frame on big rigs, but it is not uncommon either, so if you look around I am sure you will find what you need for a price you can afford.
I believe there are a few on here you can talk with about that, including Jay Shapiro, builder of the Eco Roamer, he lengthened his frame by about twenty feet.
Cheers
Brian