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I would say probably 12:1 depending on the parts and how much timing you throw at it. The tuning window will be relatively narrow, so make sure you have a skilled tuner working with you.
If you keep the dynamic ratio at 8.5 or less you can run 91octane,
8.5 to 8.7 93 octane, with the higher static ratios, plus good tuning, and other factors. Lots of good info on the forums on how to do this.
I went 12.3:1 on a forged LS6 before but I eventually started using E85. Gotta be careful with timing. 91 sucks so I'd probably go 11.5 max with the right heads cam combo.