With that many miles on the engine and assuming the timing chain and gear are original, the problem is likely a broken timing gear. What year did they stop using the nylon coated timing gear on the crankshaft? I just went through this same issue with my Poly 318. It was easy do diagnose by removing the distributor and spark plugs and then rotating the engine back and forth manually roughly 20-30 degrees. If the gear set and chain are intact, the rotor should rotate both directions immediately as you rotate the crankshaft by hand. When the gear is broken, there will be a delay in rotor rotation when you change direction of crankshaft rotation.