Complex sentence: Designed for the short, 200 mile, races of the sports car series, it became indominable and proved that by also winning the 1965 12 Hours of Sebring on one of the roughest tracks in North America.
Split into simpler sentences: Designed for the 200 mile races of the sports car series, it was almost impossible to beat. It proved that in 1965 by winning the 12 Hours of Sebring on one of the roughest tracks in North America.