As I remember, the header construction job was quite time consuming. I would estimate it took close to 150 hours, from opening the Hedman box to starting the car again, plus modifying the shock towers.
However, the towers were more of a “hack out and weld back not-thinking-so-much job”.
What made the job (header construction) more difficult was the 2” diameter tubing (really hard fitting them in, although tower-hacking), and the fact that I still had certain parts left of the shock towers (I still use the stock
suspension). It is actually nice to be able to change the spark plugs when you are finished, which is easy to forget during the build process. ; )
If you are going for turbo (Nice! I wish I could afford that) you will probably be using something around 1½” header tubing. That will give you plenty of space, especially if you have completely removed your shock towers. If I would repeat the job with smaller tubes, it would definitely take less than 100hours.
With smaller tubes and more time spent in engineering, shock towers might not even need to be modified (at least not as much as I modified mine).
PS. I really like the specs of your fastback!