Randy Forbes truisms:
The Z3 suspension carrier is NOT the same as the M version. The M has an internal brace not present in the Z3. The Z3 version has different geometry for the inboard trailing arm mounts, increasing static negative camber (dramatically increased with lowering springs). Weld-in toe/camber kits can be biased on installation to remove the negative camber on Z3s.
The rear swaybar mounts, both the tabs on the trailing arms, and the mounts for the bar on the body, are prone to failure. I've found them completely detached even with the stock rear bar. There are commercially available brackets to weld onto the trailing arms (though fabricating your own is plenty easy too) and boxing in & securely welding the body mounts is a relatively easy task too.
So far, we have two (2) good reasons for getting that MIG welder on your wishlist__and DEFINITELY go with the shielding gas, NOT flux-core wire (which looks like birdpoop on it's best day...).
And for today's finale', the primary subject at hand; getting those #&*^#$@ bolts out! Okay, this is a foolproof trick__(secretly hoping that I didn't just jinx it...)__but it works for me time after time after time, particularly on the rusted/seized nuts/studs that secure the mid-pipe to the exhaust manifolds. It's going to take some heat, if you can get to a dull cherry, you'll achieve enough expansion/contraction to break the molecular bonds of rust. A MaPP Gas portable bottle/torch is adequate for this, and available at most any tool supplier or hardware store. Let it cool, and touch a small stub of candle to it. The heat will wick its way into the threads and help lubricate them. This can be repeated a couple of times. Ideally, while still hot, you can get a tool to hold the bolt, and begin to work it back and forth. If it stops turning, reheat, and touch more wax to it and try turning again.
Okay, that was Round 1 on the bolts, but that stood its best chance of working before the head got all b*****ed up!
For Round 2, roll up your sleeves and let's get 'er done! Cut your losses, and DRILL off the bolt-heads! You have the center of the Allen socket as a guide for the center, and drill progressively larger holes__USING ONLY SHARP, HIGH QUALITY DRILL BITS__untill you get to the tap-size for the M8 (I'd have to look it up, but maybe a 6.5mm) by which time the heads should've fallen off.
Get the suspension carrier out of the way, and then finish the reconnaissance work to restore the threaded holes in the body. IF there are still remnants of the bolts left in the holes, the WORLDS BEST EXTRACTORS are Left Hand drill bits! Ask any aeronautical technician or engineer (not to mention WISE auto techs). The heat from drilling, and the cutting-biting action is all but guaranteed to walk the stub out of the recess before you can ever drill all the way through.
ALL of these things that I've described, can be backed up with photo-documentation, ANYONE is more than welcome to peruse my gallery of work albums (like every car that I've worked on in the last twelve years has its own album, currently at 61,807 images).
I good for some things 
