He ran down the field at Talladega, under green, from five miles back and no one, to my knowledge, has ever cracked the mystery. Really want to know.
If you haven't read the attached articles, it's worth the time. Especially if you're new to the sport, i.e. within the last 25 years. Widmer posits that the Elliotts ran his heads and pistons. They, and Glidden, denied it, vehemently ("me think thou protest too much"?) . Bud Moore's son Greg is interviewed and says that they ran them because they heard the 9 car was using them and that the "Widmer combination" did exactly what Larry Widmer said that it would do.
These days, newbies to the sport think thirty car drafts at the super speedways have always been the case. Nope. The big track used to be just like the rest: A five car or so pack at the front followed by a ten car pack 2-5 seconds back, then the rest. But even from the time Daytona opened the draft was a factor. But not that day at Talladega. Never has there been a car capable of (and demonstrably) running, alone, 3-5 miles faster under green than the rest.
The really funny part, in retrospect, was watching other cars passing Bill early in the race, before the oil line mishap put him in the pits.