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I happen to also have an 06 GTO, and am in the process of adding the 3.91 gears to that car.
If you want to go any deeper than 3.91, you will need a new case. They are out there, but expensive. These are Direct drop-ins for GTO, and I assume they should fit into the SPF (but YMMV). Strictly speaking, the difference between 4.10 and 3.91 does not justify the cost of having to go with a different diff IMO.
I second the gforce engineering referral. They are great, and have all the parts you should need. I've bought a bunch of their stuff and it's all quality. They sell the gearset by motive. They also sell center sections and master rebuild kits for those holden diffs.
For the record, I have 4.10s in SPF #408 and I really don't like them...I wish I had 3.70 or taller, but that's my car...
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