CMC uses the heavy steel sub-frame to get "hot" air to the front of the car to include a pipe for defrosters. If the sub-frame were cut in the center it would be weakened. The 356 uses a unibody welded construction.
To be honest, not that much heat makes it forward - the steel pipe is not insulated plus airflow is not boosted with an added air blowers (which would help). Some have re-piped air into the center tunnel with positive results.