Did this engine come with a harmonic balancer and timing tab or did you have to supply one? There are several different locations for the timing marks on Chevy balancers depending on where the timing tab is located on the engine. You need to verify that the TDC indication you have is correct. On an assembled engine you can use a piston stop in #1 cylinder. Turn the engine by hand in both directions until it stops, mark the balancer in line with TDC on your timing tab in both directions. Halfway between these two marks should be true TDC. If you have the wrong pointer for your engine or the balancer hub was marked wrong, your timing can be off a LOT and still look right with a light. A quick way to check would be to just crank in some more initial timing and see if it helps.