I've seen pictures where guys have cut out the crossmember arch that goes over and across the tranny,
then they tied the two cut "ends" together with a reinforced straight piece, under the tranny.

Does anyone make a bolt-on piece that does this ? I'm talking B and E body. A-body I don't know.

You would need to do this if your putting in a physically bigger tranny and need to cut out the tranny tunnel.