If you wanted to make one, you should probably start off with a wagon. Most factory utes (ie. Commodore ute, Falcon ute) have started off as wagons then got chopped up to achieve what they want.
This means that you are going to have to do a shitload of bracing to keep the structural integrity of the car to a standard, not to mention getting the bastard of a thing to pass roadworthy etc....
Easier would to be make a one tonner style thing, mounted some chassis of another smaller car, ie sierra or something.. but in reality its a headf@$k and alot of work..
If you want to do it though, go for it!
Dan.