In 1959 GMC introduced a unique 60-degree heavy-duty 305 in3 ( 5 L ) 60 ° V6 for use in their pickup trucks and Suburbans , an engine design that was later enlarged to 478 in3 ( 7.8 L ) for heavy truck and bus use .
The engine was later enlarged to 478 in3 ( 7.8 L ) for heavy truck and bus use .