Alright, let's think about this a minute. You want to extend the bed from behind the rear wheels, without extending the wheelbase?
First of all, that may look a little funny. You're basically going to have an extra foot or so sticking out from behind the wheels. Your Bronco is going to look like it has a huge butt. Then there is the wieght distribution to consider. Now, on level terrain it may not matter, but if you are off roading and going up a steep hill, your truck may want to tip backwards more because of the extra foot. If nothing more, you aren't going to be able to go up as steep of an incline as you are now with the stock length.
Besides, why spend all that money? If money means nothing to you (but I'm assuming it does or you would be driving an Excursion or something newer and larger) why not buy a brand new F-150 or F-250 and turn that into a new Bronco? Cut the walls out, buy a cargo shell and put in a second seat? At least you wouldn't be messing with the wieght characteristics of the truck, and everything is new.
Or, buy a four door Bronco. (www.fourdoorbronco.com) It'll give you the extra room, you can still take the cover off, and you're still driving a Bronco. It'll probably also cost you a ton less then having all the custom body work done. Keep the 2-door for fun, and use the 4-door for moving the family, cargo and whatnot around. (And if you really get crazy with the 2-door to the point where it is no longer street legal, you can use the 4-door to get it to the trails.)
For the gain of maybe 8 inches (because let's face it, that's all you'll really get) it's really not worth it. Buy a 4-door.