I have had my 3200GT lowered (15mm - Assetto Corsa spec) but it was as part of a complete AC exercise as suggested by Davy where I swapped out the springs for the AC items (surprisingly half the price of regular springs), an AC steering rack, the SM ARB and finally a full precision geometry set up but to AC specs which are slightly different to a standard 3200GT. As Tim suggests, it was a matter of having to remove the struts to move the collars but as I was changing the springs they had to come out anyway.
What I did find was that when the car was lowered, the AC springs fitted along with the AC rack but I was still waiting for the uprated ARB which was delayed because of problems with the bushes, the handling was a bit of a nightmare. Once the AC ARB was eventually fitted it was absolutely brilliant - I suspect the problem was the stiffer springs rather than the lowering but obviously I can't be certain, the rack probably wasn't a factor in this temporary mismatch type problem. In this half way state for a couple of months, the ride and the roadholding can only be described as "choppy", it took the uprated ARB to bring it all back under control.