Complex sentence: Three months later NASCAR President Bill France, Sr. agreed to sanction Winston Cup Series races at the speedway upon completion, marking the first time NASCAR has made a commitment to run a race at a track that had yet to be built.
Split into simpler sentences: Three months later NASCAR President Bill France, Sr. agreed to sanction Winston Cup Series races at the speedway upon completion. This would mark the first time NASCAR has made a commitment to run a race at a track that had yet to be built.