Sounds like they gave you the wrong starter. I thought the minis were all interchangeable. But I’ve been wrong before. You’re using the original trans and converter to the car. The only thing I can think is the register in the crank where the converter goes isn’t machined or possibly is the wrong diameter and pushing the converter out maybe. One other thought is the flex plate installed correctly as far as the proper side facing the block. Just some random thoughts on my end.
