I think everyone is reading more than what is being said here (till now).
He only says the timing mark is jumping around, not that the engine is running rough or misfiring.

A lack of dist.shaft collar will never cause such a large timing deviation, unless the distr.shaft 'tang' is way too short.

If the advance springs are too light, the engine's idle will be nervous as well. No mention of that yet.
Since it's a rebuild, I would almost presume the original distributor is reused.