These are pictures of each side of rear of my LX. Using 1995 SN95 GT springs in the rear. Driver side can fit about one finger, while my fender gap on the passenger side is about 4 fingers. Pretty big difference. Is this just a matter of one side settling more than the other? Or maybe one side not settling correctly?

Which way did you orient the pig tail on the springs? They have to be the same or you'll get different ride heights side to side.
Had someone install them unsupervised.
Well if it was even side to side before the new springs there is a 99% chance it's an installation issue.
Are you 100% sure that both springs are from a 95 Mustang GT? Correct installation of the rear springs is detailed here on page 2:
http://www.maximummotorsports.com/assets/install/pdf/rear_susp/MMRLCA-1-1r2.pdf