Suspension Ordered new leaf springs... Question...

Checked the listing. It says they are standard production springs. Personally, I would try them out with the supplied shackles first. If you need just a skosh more, use air shocks. Long shackles affect the handling in some pretty ugly ways that are not always apparent until you get out of shape on bad launch or have to make an emergency maneuver in traffic. Also, rear sway bar FTW!

What do you mean, "rear sway bar, FTW!"?
You think sway bar good, or bad?

I don't like extended shackles, however I would re-use them if needed. The rear crossmember where the shocks are mounted was not designed to support weight, it was designed to dampen suspension movement. Most people who have had to repair this area have an extreme dislike for air shocks or coil-overs. The factory stance on these cars is a bit lower than your pic. Other options could be to add a leaf on each side from your old springs, or possibly take them to a spring shop and have them put a bit more arch in them. Good looking stang, and good luck with the springs!