Slant Six Tuning/Running Issues

Update: I checked each vacuum circuit with no found issues. The internals of the distributor look great, almost new. Set the reluctor gap. I have not been able to hot lash the valves yet. Reset the timing to 10° BTDC. Since I have reset the timing I have yet to have any major issues with drivability. I have driven the car about 200 miles on about 8 different occasions.

With that being said, how far can you push the timing on these? I see that people are running 14+° on some. What is the best way to go about finding this threshold?