It's possible you've got a mismatch between the timing mark on the harmonic balancer and the one on the timing cover. Early small block timing marks were about the 2:00 position on the timing cover, while newer ones moved to around 12:00 with a corresponding re-position of the line on the balancer. If you've got an early (or aftermarket) timing cover combined with the late-model balancer, the marks will be WAY off. Ditto for a new timing cover and an old balancer, which would result in the badly-retarded timing you're describing.
Jerry
