[Bluestep]

To start with, the engine in the video was using studs and nuts to hold the starter on. This is not correct! To register correctly you need to use the knurled bolts that Chevrolet used. Most parts houses can supply them. Use the correct starter for that year of block and use the bolts that match that starter. At some point GM went to metric bolts so the bolts and starter need to match. In other words metric bolts with metric starter, imperial bolts with imperial starter. The bolt shoulder should fit snugly in the block's countersunk holes and it should fit the holes in the starter snugly. If the bolts are loose in the holes in the starter, the starter can move around and will not fit the same each time it is loosened and retightened.
Get a new starter or a new nose piece, as the rebuilt ones have been remachined to clean up the surface and they will require shimming.
Millions of these engines were built and operate without problems. I believe the problems start with rebuilt starters, aftermarket flywheels and people trying to reinvent the wheel.