Basically, I cut off the front wall, under the cab over bed, shortened the length of the camper body by one foot, and reattached the front wall with L-brackets until I was ready to have it welded. (More on the welding later). That gave me a 36 inch bed over the cab, and it will have a 1 foot slide out inside for a total of 48 inch width by 78 inch length for the cab over bed.
I've wondered about the size of the bed myself. I'd love to not have to pull out the bed at all, AND have it be wider than 48" for sleeping two shorter folks East/West. Have you thought of making the bed wider (even 6" would be nice - even though it is not quite Queen Size) and having cabinets under the over hang?