As long as the starter case is grounded to the engine block that should work. It is more usual to ground the battery cable to the engine block, as Don points out, but the starter case needs to be at ground, too, for the starter to work.
The only issue I see with the ground cable attached to the starter case is what bolt was used?? Unless it was one of the starter-to-block mounting bolts I feel any of the remaining fasteners are too small for a good connection.
Thanks Don, I sanded the rust and paint off an engine mount and moved the ground to it. The starter still turns a little slower than I think it should. I probably need to replace it or have it rebuilt.
