I'm almost 100% certain the mount lugs on the S197 and S550 are the same. It's just the height of the calipers that is different. I started out with S197 caliper brackets. All I did was moved the caliper mount holes down to work with the 14" rotors.
Could be. I just keep seeing that the S550 6P caliper needs the 13/14 GT500 15" rotor rather than the S550 15" rotor on an S197. Makes me think they are different. The other thing I don't know is if the spindle is different between say a GT with standard brakes and a Boss 302.
FYI, looks like Full Tilt Boogie Racing has brackets to bolt GT350 rotors and calipers to S197 spindles.