These should fit under 15" wheels, as 300zx calipers do.
You are not going to strain the rotors in any way shape or form. Tires loose traction before you ever come close to worry about tearing rotors apart. Next comes wheel studs.
Keep in mind that some Q45 brakes are smaller during certain years. I would suggest j30 calipers, as they have the same size pistons as the q45, but the caliper is much smaller.