Complex sentence: Popular in the United States during the early 1960s these models were replaced by fullsize vans, at the end of the 1960s in this market, these used body-on-frame construction and featured front engines under a short hood.
Split into simpler sentences: Popular in the United States during the early 1960s, they were replaced by full-size vans at the end of the decade. These large vans used body-on-frame construction and featured front engines under a short hood.