Kostecki took the lead from front-runner Alice Buckley in the opening laps, the pair leading home series regular Tyler Cheney in a close battle to the finish.
Cheney charged through to the top three as front row starter Ben Gomersall slipped back outside the top three in the opening laps.
After a tough weekend so far, Cam McLeod got away to a fast start, charging through to top five.
McLeod came unstuck at the mid-point of the race, sliding off the road at Turn 4 and dropping back to fifth spot.
The Ford Racing junior worked his way back into fourth spot by the finish, ahead of Gomersall rounding out the top five.
Pip Casabene continued the quick speed shown so far to finish sixth, ahead of Brock Paine in seventh.
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Tommy Smith bounced back from a tough start to the day to finish eighth, while Joel Heinrich and Jackson Rice rounded out the top 10.
Following damage sustained in the earlier race, the cars of Mark Crutcher/Hugh McAlister and Steven Wilson Nick Lange did not start the final sprint race.
The TA2 Muscle Car Series returns for the second and final 40-minute enduro at 4:10pm AEST.
The Chevrolet Performance Camaro of Buckley and Tim Slade will start on pole position, hoping to reignite their dominance shown earlier in the weekend.
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