And you are sure the bolt holes in the starter are the correct size for the 7/16 bolt and stud. Many of the mini mstarters have a larger hole and require a bushing.,
We had a Race truck engine come back due to a similar problem. It destroyed the rear main after a couple races. We asked the customer to bring the converter to the shop. After chucking it up in the lathe we found the pump drive hub was welded on crooked. At the ring gear it was over 1/2 inch out. The converter came from a reputable trans builder always mentioned on this site. It was overlooked when built and shipped.
I found it is very important to check all parts being used when assembling everything today. Quality is not what it use to be. We can blame it on what ever. But it always comes back to the bumpy ride crossing the ocean. The waves are getting bigger due to climate change.