LA motor with stock iron heads with the spark plug in the stock location I wouldn't run much more than 9.5 to 1 compression ratio with a stock type heat range extended gap plug like the OEM Champion RN9YC or the smaller plug socket size like the RC9YC with no more than 34 degrees total timing all in by 2200 RPM twocents
If your near sea level elevation than start with 30 degrees total advance and go from there up scope
All the 1968 to 1971 stock factory assembled 340 motors I took apart barely had 9.25 to 1 to no more than 9 .75 to 1 compression ratio when we did the math on those OEM factory built motors work

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)