ok, here is "the rest of the story" ..
The Auburn 541017 is in fact a spring retainer kit with three large (redblack) and two small springs (yellow) and they fit the stock diff of my 98 Camaro.
However in the end I got myself an Auburn Pro Series diff from Ring and Pinion in Everett, WA (btw check out Tate Hudson there, great service, fast delivery, 2 days later UPS showed up with the Diff).
http://www.ringpinion.com/Default.aspxWhoever owned this car before stuck his own self-made shims in, which means, we simply did not know anymore which the original distances were.
This also meant that you would have to have gauges to measure in your differential along the cam shaft axis and the axle axis, which we did not have.
So the best (unfortunately not as economic) solution found was to get the car to the Opel/GM workshop in Frankfurt and have them do the job.
Fortunately I also had a ring and pinion which fit the new differential, so in the end all was replaced: all bearings / shims / diff / ring and pinion and the whole thing was measured in anew.
Pricetag: EUR 800 for work and measuring that stuff in. USD 470 (diff) + USD 200 (UPS 2 day shipping) + EUR130 (customs) => ALL: EUR 1500 more or less
However, I ended up with a new rear, which feels quite good now.
BTW I also upgraded the air intake. Already had a FTRA Kit and now I stuck in the Fast Toys Air Lid and an SLP 85 MAF as well as the SLP smooth bellows. I also moved to the Hypertech 160 Thermostat and all together you notice a considerable power and traction gain.
So I guess the standard tuning mods are done now, and I can start putting cash aside for the more serious upgrades ...
Best
Pedro