We have some 1.5mm aluminum front shims and some 2.5mm rear shims. These in conjunction with the thicker OEM shims, we usually find a combination that works.
However, with the adjustable perches - lowering the front about .75" and the rear ~1" will typically put the GT in aggressive-street set-up with the potential of camber in the rear being just a tad aggressive. However, noting that 95% of GT owners tires are replace on age and not wear, the rear camber is mostly a don't care.
I answered this some years ago in another thread on alignment. You just need washers that are of certain thicknesses and you can stack them up to get the adjustments you want. I went on the expensive side and bought a couple of the plastic packs of stainless steel washers at Home Depot for about $4.00 per pack of 4 or 5--I forget the exact size and quantity. Just take one of your shims off the car and go to your fave hardware store and get some the same hole and diameter. The SS ones are pretty consistent thickness (unlike some the galvanized steel), but use a precision caliper to measure the thickness of the washers, then stack 'em up to desired thickness. Simple, cheap and effective. 3 of my favorite things.
I like Pete's idea but I would look for aluminum components to avoid the use of dissimilar metals. If you can't find anything local, we have a kit including (4) front and (4) rear aluminum shims. These, combined with the OEM spacers you already have should allow you to find the best combination for your set-up. Ford GT Shim Kit