Pony Xpress was made by Silver Crown luxury coaches, which was owned by Supreme Industries. Supreme got out of the RV business in 2010, so no more Pony Xpress.
Have you looked around on Racingjunk.com? I think you would be better to find a toterhome ready to go. Sell the F450. You're going to spend lots more having your truck built. And I don't think you could build much for living quarters onto that F450. You're going to be overloaded before you put your luggage in.
There are a few out there. The company was started by ex Renegade people, very good construction qualityt still not the towing capacity my customers want.