Longtime Alfa Romeo TZ driver Marnix Dillenius wheeled #76, a 1960 Ewing Indy Roadster, to 3rd place.  Larry Menser finished 4th in his Meskowski Rdster .

Regarding the Formulas:  In the 1930s Grand Prix cars were permitted engines of 4500cc normally aspirated or 3000cc supercharged.  In the 1950s USAC modified this formula to 2800cc. supercharged or 4200cc. normally aspirated.  The last I heard CART had dispensed with normally aspirated motors altogether and now limits blown units to 2600cc.