Correct. In early stages of development they had aluminum in mind, and so specified the block geometry to be adequately rigid and strong in aluminum. It became apparent they wouldn't be able to finish the R&D on design and production methods for an aluminum block in time to launch the new Valiant and the '60 Dart and Plymouth, so development of iron and aluminum versions proceeded simultaneously with the iron one released in late '59 for the '60 model year and the aluminum midway through the '61 model year.