You only "need" caster/camber plates if you lowered your car more than approx 1" moving you outside of factory spec, and then only if you didn't use camber bolts, such as these:
Eibach Mustang Camber Adjustment Bolts (05-14) 581260K - LatemodelRestoration.com
While your problem likely is the factory strut mount, it's not because they're bad, or at least they weren't when you started the process.
What I've found to almost always be the case is that if you are not careful when letting the spring decompress while putting the strut assembly (strut/spring/mount/strut nut) together, then the lower half of the strut mount will pop-off/separate and one or two things happen:
1. upper/lower half of strut mount are no longer snapped together and aligned and are not riding on the ball bearings/races between the two halves
2. one or more ball bearing popped just outside of the bearing race, and is jammed between the upper and lower halves of the mount, causing the actual strut mount halves to grind on a estranged ball bearing.
Either condition will cause the noise, and the only way to fix is to remove the struts, disassemble, inspect, and then reassemble after correcting. If you lost the bearings between the upper/lower half, you will need to replace the strut mount.
If you re-used the 2011+ Ford OEM strut mount and strut, then you cannot use a GT500 mount as suggested, only a OE Replacement from Ford or an aftermarket set of caster camber plates such as these:
Steeda Mustang Heavy Duty Upper Strut Mounts (11-14) 5558135 - LatemodelRestoration.com