Gotcha. Mine’s the Mercury obviously.
Maybe I am counting # coils wrong, but it looks like your Mustang spring has more like 7, not 9 as shown in MPC for the 4E spring?
As is so often the case one assumes that what you are seeing is a stock item when in actually it got replaced at some point in the past. I also got out the digital calipers and it measures .620 wire thickness. So fewer turns equals a stiffer spring and thicker wire is stiffer. The code 87 spring was rated at 1720 lbs. The 4E was rated at 1650 lbs. If this was a replacement for the 4E it might be about right as the aluminum heads, intake and AC compressor plus the HiPo exhast manifolds are probably good for about 70 lbs of weight reduction. I guess I will find out.
I have found the MPC spring specs useless in helping decide what spring to use. They do not specify spring rate! The Moog catalog has spring rates but they do not match the factory springs. Moog springs (may) have the part number stamped into the spring for identification.
As Royce suggested, cutting the spring to lower the height is most effective. It is time consuming but do not cut more than 1/3 coil at a time (don’t ask me how I know). You don’t have to fully assemble with the shock to test after each iteration. One side at a time works. Document at each point.
I would start with the blue “620” springs. They worked out of the box on a '67 289 that I built, along with old sagged 1969 rear springs.
Note that your front bump stop may no longer hit the upper control arm in the correct location with Shelby/Arning drop - test with the spring/shock out.
You can also take some photos from the side and have them printed…carefull cut the car off the wheels and drop the body down to desired stance. So during your marithon coil cutting , you’ll know when you get to where you want to end up.
Update;
I put the Mustang front springs in. When I actually put the digital caliper on it the wire was a little over .620 (paint thickness?) So that made it .020 larger than the stock spring. It is also shorter, and that also tends to make a spring stiffer. (shortens the lever arm total length of the spring). So…
It worked! I have the slight nose down rake and good bounce and rebound. If it settles a bit more I won’t be unhappy. Ride height is about like stock, which with my crappy road is not a bad thing.
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Sounds perfect - good call!