Lean burn with advanced timing is asking for major trouble.
I like to run a stock piston 3sgte with no leaner than 11:1, on my own car the leanest i gets is 10.7 or so.
under no circumstance should the afr get leaner than 14.7:1 on this engine, from my own experiments and experience the torque significantly drops past 14.4 or so AFR. under cruise. What i would do is drive the car on a steady cruise, and slowly lean out the mixture while maintaing a steady speed and load(best on the dyno). I could clearly feel, hear, and see(coolant temp) that the engine was not happy with anything past 14.4 during loaded cruise.
I had trouble getting a steady idle with a lean mix, so i stick to about 13.5-14 at idle. little richer when cold. around 17 or so degrees of timing with my set up. Remember that fuel is for thermal management, not just getting the car to move. so a richer mixture is going to keep things cooler, leaner is going to make it hotter.
it is very important when asking questions about what would be a safe timing/fuel mix that you state your mods in detail. Where you could run a ct26 vs ct20b vs 20g safely is very different
hope this helps