The 29-year-old Chastain from Alva, Florida, rallied from an early pit road speeding penalty to overtake Erik Jones on the final lap through the tri-oval to claim his second career win in NASCAR's premier series.
The 23-year-old Gragson from Las Vegas stormed to the lead during the third of three overtime attempts before holding off Jeffrey Earnhardt to grab his first victory at Talladega.
The NASCAR Cup Series and Xfinity Series travel to Talladega Superspeedway this weekend. The ARCA Menards Series will also complete Saturday afternoon in the General Tire 250.
Denny Hamlin finished seventh at Talladega in a rain-delayed and rain-shortened race won by Bubba Wallace, who drives for 23XI Racing, co-owned by Hamlin and Michael Jordan.
The Mobile, Alabama, native fended off the field in the closing laps of the second stage to become the second African-American competitor to win in NASCAR's premier series.
It took one overtime and one wild last lap for Tate Fogleman as he collided with John Hunter Nemechek coming to the finish line to capture his first career Truck Series victory.
