Hi,You can use GT350 springs but only the front struts. You cannot use the rear shocks without also swapping the control arms (they have different lower mounting geometry). GT350 springs with Ford Racing Track handling dampers are a great combination. You have to use two left rear springs to fit the GT, since they also have the counterwound right springs.
I have GT350R front springs on my car. The drop is ~0.7". The 350R rears drop the back about 0.3".