The term you're looking for is "track width" ("wheelbase" is the measurement from the front to the rear axle).
It can be/has been done, but it's an ambitious undertaking. How wide of a tire/wheel are you trying to get under the fender? How much clearance is there now between the inner sidewall and the control arm/control arm-to-springplate bolts? The upper shock pad can be trimmed easily, and some clearance can be gained by altering the control arm hardware. This may be enough, if you select a wheel with more backspace.
If that still doesn't do it, you've got to narrow the control arm and use offset springplates and shorter axle shafts.
...What you do to narrow the arms is cut the bearing housing free from the arm, cut the arm back, set it up square in a jig and reweld. Bruce Tweddle wrote a How-To article in Hot Vws back in the early 90's. Another way to go would be to buy some tubular narrowed control arms from either aircooled.net or Blind Chicken racing. The AC.net ones are 1 1/2" narrowed and blind chicken will make whatever you want.