Judging from some of the specifications, Mustang wheels should fit fine on a Z34. The PCD is the same and the hub bore is larger. Make sure that brake calipers would clear and proper lug nuts are used (Z34 uses 12x1.25 and S550 uses 14x1.5). S550 wheels likely use cone-seat lug nuts and if your current wheels are not OEM, they're very likely already use proper lug nuts to fit the S550 wheels.
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