Starter Problem

It looks like the ring gear on the converter is a little worn from the problem. Mostly in taper on the ring gear, but the problem is because of the starter sitting too far away from the ring gear.

The starter gears are tapered to help engage the ring gear, but after watching your video a few times, I think the gear isn't disengaging (as shown on the last revolution in your video) because it's binding, from a shallow tooth engagement. The gap between the gear of the starter and ring gear is too far.

At this point, I would start looking at the starter as the culprite. I think you have an indexing problem with the tip of the starter being too small or you may have an issue with the indexing cup in the bellhousing of the transmission being out of round or too big. Either way, there is something keeping the starter too far away from the converter ring gear.

If you are able to look at the top of the starter gear housing index and measure it and get a drill bit or dowel that measures the same size as your starter and use it with a flash light to check the cup in the transmission, that can tell you if the starter index is loose.

It is also a possability that the starter index casting on the starter itself is off. If you have an original starter around, I would urge you to measure the bolt pattern distance, from the center of the bolt holes to the center of the starter index and compare it to an original. Even if the index on the tip of the starter is the same size, it could be off.

If all of that checks out, meaning your small starter has the same measurements as the original, the cup in the transmission looks good and the index diameters match nicely between the starter and cup, you may have to replace the ring gear on the converter or get another converter to correct the problem.

Did you get the engine and converter together, from the same person that told you it was difficult to start?