I would insert a positive stop into the sparkplug hole. Turn the engine in both direction to contact the stop and mark crank position. Find the spot 1/2 way between those marks. This is TDC. Then verify on balancer and the timing tab. This is where to start.
GM made balancers with timing marks in different places and timing covers also with tabs in different places. You could have the wrong mix of parts.
Your almost there.
