I think you need a deeper rotor or different wheels. I doubt the GT500 calipers will be any smaller. I could be wrong, but just betting there isn’t much of a way the calipers will be smaller on the front side.
Someone on FABO used the same GT500 calipers and rotors, found the tread last week. His bracket came off the back of the spindle and had a very deep cut to match everything up. I would use a bracket that comes off the front of the spindle instead and only use a cut on the bracket if the available material is too thick.
I like it though. Keep at it.