Sounds like the external ring on your harmonic balancer has turned, not uncommon with old, stock harmonic balancers. As said previously, put your engine at TDC on #1 cylinder, check the rotor to make sure it is pointing at #1 plug wire. If the timing mark on the balancer is not lined up with the pointer the external ring has slipped and you will never be able to get an accurate timing reading until you replace or rebuild the balancer. HEI is a drop in, works fine and lasts a long time.