Both of my 340s are 10:1 J head motors, one is 268 the other 274 cam. My 67 is street/strip with an aggressive ignition curve 27*/34* and runs great on 93. The 73 is street only and runs on 89 with the 268 cam. A good tune is important, you check for detonation with a magnifying glass and look for specks on the porcelion. Run it on mechanical timing only first, then add the vacuum advance and dial mid to upper 40s for cruise. I use ported on the VCAN. I run MSD 6AL boxes and Autolite copper core 65s at .035 gap. A good phenolic spacer also helps to keep the carb cooler. Both cars have functioning sealed dual snorkel scoops for fresh air.