You can only trust the factory timing marks if you have verified that they are in the right place.
Lots of room for error here.
1) many times the rim of the dampener, which has the mark on it, has moved in relation to the hub because the rubber between the two is not bonded anymore.
2) several different timing covers have been used over the years with timing tabs/marks in different locations requiring the matching dampener.
3)you don't know how the builder selected these related parts.
The most accurate way to verify that the marks show top center is with a piston stop which you will have to fabricate and we can talk about that and the way to use it---but if you use care you can remove the plugs to allow easy rotation BY HAND and feel with a wire or small screwdriver for the end of piston travel through #1 OR #6 spark holes.this should be close enough to determine if the TC mark is close to right.
Also can pull a valve cover and watch #1 or #6 valve movement during overlap position, knowing that when inlet and exhaust are open equal amounts ( the exhaust is closing, the inlet is opening)this SHOULD BE TDC for that cylinder therefore the marks should be lined up.Observation of this overlap should show TDC within 4 or 5 degrees.
If the dampener rim has moved you could make your own mark but many times it is only a little way from flying off altogether many times leaving some things in the vicinity with some battle scars.
