3d solidworks or any other file type you have iges/stp would work.
Has to be an accurate model to give accurate results though obviously.
If your mods are purely aesthetic for you then don't sweat it but then also lets not presume it's going to be 100%. or better than the original starting point.
Enjoy
Ok. One of my car group buddies works for a company that writes this type of flow software, and he is trying to get me on as a beta tester for it. I have one of their really nice 3d programs now, but my inexperience in the program, or most things that happen on a dam computer, I have since only made a cylinder with a beveled edge in an hour. Hahaha. Scanning would have to be my option as yes, totally agree any variation from actual will show drastically different numbers. That worries me since part of it crests the top of fender, which puts different parts of the piece in vastly different airflows if not modeled perfectly. Because of it's location, I would really predict flow characteristics to transition back and forth between the two variants. One better at one speed, other better at another speed.
If this has come off as any type of competition or anything, that is exactly opposite of what I was trying to achieve. Numbers that I use, I only use as baseline numbers (to gauge my own ratios) and not meant as a competitive comparison. Purely just building my own car to reflect my tastes and the fact I have SERIOUS automotive OCD and can leave nothing untouched. Lol. My build motive really has been a balance between astetics, and function (what I feel will work atleast), with never trying to let one vastly outweigh the other. So anything i leave on the table design wise aimed at strict performance, was done to create the correct astetics.
I would really like to bring the whole front and rear package to you if I could catch you guys on a practice day. The front should set right on, and will keep in mind retrofit parameters on the rear while building it. Many months away, but keep in mind if interested.
If it works, it works. If it doesn't, I will hit the drawing board again, no biggy. I will just keep going here, so I do appreciate the feedback and participation recently in the process.