1. do you have an early small register hub converter from a pre 69 trans with a large register crank that all later engines have?
2. Your flex plate, Does it have holes for 7/16 converter bolts and your using a 5/16 bolt converter.
That would be the first questions I would have.
Also did you know that some of the mini starters require a bushing to be used on the top bolt. Some of the starters come with the bushing and some don't. Many times this is over looked.
My guess is you have a small register converter with the large crank hole . Also together with the large bolt hole flex plate with the smaller 5/16 converter bolts.