Custom CV Axles
They press the yoke off, cut it to length and broach fresh splines on before pressing the CV yoke back on. You may want to inquire about changing the yoke to match the CTS diff as its unlikely the Miata CV's had the same spline count
If that's a no-go check with import and corvette speed shops. Ask them where they get theirs. You can change directions and use half-shafts instead of solid axles, similar to how C4's did it. Those literally are half a DS, so any DS shop can build one to suit



