When you re-install the (new) compressor, first check to see if it sits flat (doesn't rock) on the four mounting pads. If it rocks, find a way to shim it or otherwise make it sit flat. Why? If it rocks and you tighten it down, it'll put a twist in the body that works to break the seal in the middle of compressor.
I've got a GM TSB around here somewhere that makes this claim, I didn't make it up myself
I've heard this one before and it makes sense, considering the design of the HT6 compressor case, its seal, and how it mounts. The TSB would be nice to have here as reference, if you can dig it up.
If it does rock, it would seem that putting the compressor on a known very flat surface to see if it is capable of mounting flat would be my first step, then adjust either the compressor or the mounting bracket if needed.
If it's the compressor I wouldn't be surprised as who knows what happens to them through their journey of removal, core process, reman, resale. Tossed around in bins of cores along the way, shipped several times, rebuilt.. imagine if simply the case halves aren't assembled with 100% precision then they could never mount perfectly flat, leading to premature failure.
If it's the bracket, plenty of care needs to be taken to be sure you get it sanded/ground down perfectly flat on all 4 points. You don't want to go crazy on it, tiny little bit at a time, checking your work frequently before continuing.
Richard