There is something going on with the installation. Not sure what you mean when you say there is no shimming on the fiero/chevy installation. All smallblock chevy starters bolt on from the underside, and you just insert some shims made for that purpose between the block and starter flange.

Now maybe the fiero swap changes that by using a starter in a different location?
How does your starter mount?

You could have 2 bad starters, but I bet something in the installation is killing them.

Don