Only difference is the clutch engagement. The Z6 tranny is hydraulic. And the one in your wagon is cable. So it's not gonna be a straight bolt in, is what I'm getting at.
The passenger side mount is different too. You'll need a Hasport cable to hydraulic conversion (http://hasport.com/store/index.php/efdhc.html), or the ability to modify the mount and convert your car to hydraulic clutch activation. Also if your car is an 88, you'll need a clutch disc with the 89 or newer spline. 88 clutch disc won't fit the input shaft on the Z6 transmission (S20).
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Personally keeping in mind the 88 disk when I get ready to swap. Sucks mine has under 1000 miles on it, and I'll be replacing it.