IMO, I don't think it would work going from 17 inch to 16 inch rims on a GT, but I could be wrong.
I've seen people put 17 inch GT rims on their V6s, but not the opposite.
As for calipers, I guess it would depend on the offset of the rim, thats if the 16s do fit.
Just my .02.
If you like the halibrand style V6 wheels, there are several people making them now. Halibrand, American Racing, and OEMWheels Direct all have that style of wheel in the larger sizes to fit the GT's.