User "oldboogie" at the forum at Hotrodders.com said,
Unless you appreciate that the crankshaft and damper pass the timing tab twice in a revolution which puts different events happening at each place because the cycles (of the cam) are happening at half the rate of the crank. So for argument starting at a point where both the number 1 and number 6 pistons are at TDC lets say this in cam (thus distributor position) is ready to fire number 1. At this same time the cam has number 6 also at TDC of crankshaft but it is finishing the exhaust stroke and will commence the intake at the same time number 1 is on a power stroke. At this point if the timing cover was to be removed the timing mark of the crank gear and the mark of the cam gear would both be in the 12 o'clock position.
Now rotate the crank one full turn, both number 1 and 6 are back at TDC but number 1 is finishing exhaust and about to start the intake cycle, while number 6 has finished compression and is about to fire and start its power cycle. The timing mark on the damper is at TDC as the example above, however, with the timing cover removed the crank gear mark is again at 12 o'clock but the cam gear mark is facing it in the 6 o'clock position.
This quirk of the SBC catches a lot of engine builders including experienced ones. the reward is an engine that seems to want to run but doesn't quite get there.
The other is the distributor being off a tooth, this is forced by alignment of the oil pump drive which rotates with the distributor any time it is removed. This drops what seems to be the TDC point by 27 and some fraction degrees. The engine will start and run but has no power and backfires a lot yet appears to be timed correctly. This also bites a lot of people.