On another forum I saw this answer to your question -
Look at the starter. If the bolts are straight across (stock gm starter) then it's designed to work with the 153 tooth flexplate. If the bolt pattern is staggered (gm starter) then it's designed to work with the 168 tooth flexplate.
I can't swear it's true, but it sounds like the guy knows what he's talking about. As an alternative, I guess you could swap your starter based on the bolt pattern, assuming it will bolt up.