Hey Brad,
This is how i would start with cutting the fender.
Every cut should be parallel to the fender main side body line that runs horizontal, mark a cut line using a chalk line and then use a length of masking tape to show the horizontal straight line and this will be the guide line for cutting that is positioned half way from the body line to the top part of the fender beauty line, then using a square i would line up where the lower body line front and rear of where the arch of the fender beauty line turns and follows the bottom portion of the fender, this should be where you stop and start your cutting. The fender will flex outwards enough if you cut it in this way to allow you a square straight section to work from, easiest way i know how.
Using 2 support rods, space out the fender cut area to allow the proper width to what you need for clearance, all of these modifications should be done while the fender is still mounted to the car to get proper shape and pattern making.
Their is a next stage after you get to this point, let me know you understand what i have written as a guide buddy
Kenny.
