[Karl Wescott]

Short of slicing and dicing on the quarter panel to make it correct here is a low tech solution.

First, do the splash apron thing and slotting thing per Eric.
Second, get the fenders as straight and accurate as you can.

Now the big trick. Wax the fender with a mold release wax. Prime the wheelwell where the fender interfaces to prep for adding plastic body filler. Slather copious amounts of body filler to the area of gaposis. Bolt the fender to the body before the body filler hardens, the excess filler will extrude out. Allow the filler to fully harden. Remove the fender (it should release easily if is waxed properly). Grind and sand excess body filler from the body. Perfect fit... just like the body was made for the fender.