I don't think it will be likely that anyone will have detailed plans for a custom box for a conversion. Anyone spending the money to do up prints will most likely be a mfg'er and they are not letting out that kind of info. The guys tackling a conversion on there own will probably be doing it by the seat of their pants, making alterations as they go.
As geof pointed out you are best to be sure you have enough truck before you get to far into this. I am assuming you will be towing a trailer and with the weight of the conversion and a trailer that 700 is going to be aching. Not sure how your 700 is setup, but I have seen them come in a 23,500 GVW & 30,000 GVW version with everything from a V8 gasser to 210 hp cummins.
I think I have my links at home and I know I have promissed before, but this weekend I will try to remember to put a new category in the link section with links to companies that supply all kinds of components to build a conversion on your own. Things like all of the metal work for the corners of the box to appliances.
2003 28' Show Hauler Motorhome on a 1995 FL 120 www.showhauler.com