Diagnosing a slow fuel leak

Bad valve timing is a good guess. You can fairly easily check for excessive slop in the timing chain by watching for dead-band in the distributor rotor movement when you manually turn the engine over in the forward then reverse direction. Should be <10 deg on the crank damper. The 318 from the factory might have had a nylon gear on the camshaft. My 65 big block did. The chain can skip a tooth on those after 40 years.