1996 Chevy Blazer Engines Harmonic Balancer Timing Marks

This engine is a 1995 Chevrolet Vortec 4.3 liter V6 installed in a S10 4dr, 4 wheel drive blazer. The Harmonic Balancer that is used on my engine has 2 timing marks. The Harmonic balancer is not marked in any way shape or form. Short of tearing apart the engine, how do I tell which is the correct timing mark to use for dropping in the Distributor at it correct timing position. I installed the distributor at one of the positions and the engine tried to fire but was back firing severly( I figured it was off by one tooth on the cam), so I repositioned the timing marks. The engine fired (but has slight missfires) the second time around. The engine now keeps coming up with a P1345 code (Crankshaft/Camshaft correlation fault). Do I possibly have the distributor in the correct position and there is something else wrong? How do I tell which is the correct timing mark on the Harmonic balancer?