I've been scratching my head over the same issue in my plans. All of the factory jobs like Bob's have that tiny little 4' bunk which is only good for kids and to say you have a bunk to make it a legal RV. Good for what it is, but I'm with you, I'd like a big bunk up there to maximize what little floor space I have. My thing is I want to have some headroom cutout like on RumRunner so I don't have to crawl my big lazy a$$ into the cab through a little hole, and still have a small bunk going forward from there. I have never seen a conversion with a big bunk. The closest thing I can think of is a furniture mover:
Furniture Trucks For Sale
that one has a 9' attic, and one farther down has a 6' attic. I think that 8' idea looks way bigger than it sounds, plus the braces are fugly. I'm thinking I could get away with 5-6' done properly with PLENTY of tubing and diagonal bracing. The whole box is screwed if I get it a little wrong and it starts to sag later, though. But we're on the same page I think.
Dave