Like Wildstang90 said. It's likely that the springs are not seated correctly.
On a
vert, even with new poly isolators, with FRPP C springs, you should have 3/4+" or more drop
from the factory spec'd height.
If the old isolators were worn, that could drop the rear by ~1/4"->1/3+"
from the car's factory new height. I'm not sure what the factory spec height is for a vert. But, it's at least 27" at the fenders. It may be 27-1/2" or more.
FWIW: I don't like the FRPP C springs. I like them even less on a heavy *ss vert. If you're doing the drop
just for looks, that's fine.
Hoeever, if you're looking for better
handling, the FRPP C annd B springs are a joke. The drop is too great, and many
suspension parameters get screwed up when the Fox/sn95 are dropped more than ~1" from the GT hieght. For a vert,
for handling, I'd suggest at most, the Bullitt or Mach1 springs with poly isolators. Or, Mach1 springs with rubber isolators.