340 has no balls
I doubt that 340 is at the factory-rated 10.5:1 compression; it's probably 10:1 at most. Take out a little bit of ignition advance and you should be able to run 93 octane. GET YOUR IGNITION TIMING SET UP RIGHT! Honestly, try to drive it the LEAST you can until you get everything tuned right because your engine (and transmission) will wear much faster, especially if everything is outta whack and not just one system.
EDIT: If you don't have a timing light, BUY ONE from a local auto parts store, preferably with the adjusting dial. They're not expensive and with a car like yours (numbers-matching 340) it would be a very wise investment.