I had this happen to me with a 400...I never came up with a solution at the time and ended up swapping in a different motor. However, I did learn later that the early small blocks (like 55-?) had the starters mounted on a "plate":
http://www.classicchevy.com/product....8&dept_id=1315
Not sure if that would work or not...I have always wondered if that would have fixed it for me. Maybe someone else on the board has experience with these?
Togart