It would cost a fortune and you'd end up with a horrible in between affair. I.E the back bumper fits the cab tailgate so you'd struggle to get anything fitted to the rear.
If you replaced the wings the lines wouldn't match the doors and the whole think would look silly.
The mk3 looks better than the mk4 cab anyway (i think)
Might be worth a closer look though as i'm sure some things would work. Especially intertior