I know of a guy who runs 10.5:1 compression in a daily driven 468 powered El Camino. California runs two different formulations of gas, winter and summer. The winter is better and he will actually run the car on 89 octane during the winter. During the summer, he has to run 91. It is only possible using a knock sensor and computer which can dial out timing if detonation begins. He runs some really high initial timing, too. Something along the order of 26 degrees. He has been averaging 25+mpg. 310whp and well over 600lbs of torque. He said he uses really long rods (427 tall deck) and a cam with a tight lobe speration to keep cylinder pressures down enough at idle to hold off detonation. Also, very tight piston to cylinder head clearences are mandatory.

In conclusion, were gonna need to know more about the engine to tell you what is safe. You should be safe, but don't be cheap and try to put anything less than premium in there.