12:1 is too much i think. Not sure it will work with pump gas. 10.5:1 should be good with pump gas. 11.5:1 should be good with good pump gas and proper tune. 12:1 would be as high as i would go with street engine. If its race only then go max compression and run race gas.
Basic rule of thumb is if you drive it to the track pump gas if you trailer it to the track then run race gas and hogher compression to take full advantage.
Ls motor is roller so there are some better options for cams. With roller and poor heads i would be looking at some cams that have wider powerband that starts low and has some punch left up top(4500rpm).
Either way i would recommend some new heads from after market. GM makes some great big heads that wprk well for all flavors of v8s. Brodix and edelbrock are also very nice.