To remove the rear springs, you just have to disconnect the shocks and the panhard rod, drop down the differential, and lift out the springs. But if the shocks you have are longer when extended than the "proper" shock, the panhard rod may still allow the diff to drop enough to remove the springs. Not a big deal until you do what is shown below, and land without benefit of any rear springs! Hard to do, unless you live in San Fransisco and are chasing Dirty Harry.
Aside from that, how does your GT ride, and does it sag on one side, or is the rear lower than the front? If it rides OK and doesn't sag, I would say that the springs are fine.