But Richard Petty, who was out of the points because of a shoulder injury suffered at Darlington in May, was considered the favorite to win the race.
Isaac, in the Nord Krauskopf's K&K Insurance Dodge, led 179 laps and took the win by six car lengths over Petty.
Isaac advanced to become the 1970 Winston Cup Champion at season's end, with Allison being the runner-up in points.
Tiny Lund, driving a 1970 Camaro, qualified sixth and led just seven laps on his way to the victory.
Dave Marcis finished fourth, two laps down, and Benny Parsons rounded out the top five.
Bobby Allison was the only other driver to lead, running 11 laps out front before losing an engine before the half.
Richard Petty and Bobby Allison swapped the lead for the rest of the race, beating, and banging each other for the win.
In the Gwyn Staley 400 of 1973, Bobby Allison landed on the pole with a qualifying lap of 21.077 seconds / 106.750 MPH.
Yvon DuHamel, a top AMA road racer from Quebec, drove a Mercury prepared by Junie Donlavey and finished in tenth place in his only career Cup race.
In the Gwyn Staley 400 of 1977, Cale Yarborough became the first driver to win a NASCAR Cup Series race on his birthday.
Only the top three, Yarborough, Richard Petty and Benny Parsons finished on the lead lap.
This ninth win of the season for Yarborough virtually locked his third straight NASCAR Cup Series championship driving for car owner Junior Johnson.
[2] The suspension on Bobby Allison's #15 Ford collapsed as he crossed the finish line on the final lap of this race.
Only four drivers would be on the lead lap at the end of this race: Allison, Petty, Benny Parsons, and Dale Earnhardt (matching his career-best finish of fourth, one week before notching his very first Winston Cup victory).
McDuffie, Richard Childress, Buddy Baker, Cale Yarborough, and Joe Millikan.
[2] At the end of the race, Bobby Allison would be a mere nine points ahead of Darrell Waltrip in championship standings, with Cale Yarborough, Benny Parsons, and Donnie Allison making up the rest of the top five.
[3] Notable crew chiefs who actively participated in this race were Joey Arrington, Kirk Shelmerdine, Darrell Bryant, Dale Inman, Bud Moore, Tim Brewer, and Jake Elder.