You really are in for a LOT of needless work to make this thing look correct and that seems to be your big concern. I've seen folks widen the rear fenders for dual wheels but it will be off the chart both in time and costs.
Suggestion: why not fabricate the body for the correct length and get some fiberglass dually fenders somewhere and go that way or just use a dually bed. You'll pay thru the nose for it but at least it is quick. Don't forget the shortening of the wheelbase to make anything fit. More food for thought.
