Rain started to fall on lap 21, four laps shy of the half-way mark.
Danny Dietrich was also on the move, coming from 15th to sixth at lap 25.
"I don't think the rain affected the track at all," Kreitz said. "I was wishing it would have poured so that they would have to call the race."
The rain let up just before the end of the half-way fuel stop and returned on intermittent basis on lap 39.
Following the fuel stop, Kreitz again opened a straightaway lead and even kept his lead in lapped traffic.
"I was really worried when we went out in the feature," he said. "When they went out and watered the top of the race track, I knew they played into other guys' hands better than mine."
Montieth took the lead from Kreitz on lap 37 when he passed him on the frontstretch.
"It was a typical run for us here at Port, We were behind the eight ball all the way," Montieth said. "We haven't been good all year, I don't even know if we are still good, but we were still good tonight, but only on the top. I don't think anybody would have caught Donnie."
Montieth won his only Tuscarora 50 in 2010.
"When Brian went by me, my heart sank," Kreitz added. "I drove around for a lap or so and I tried to pick the pace up. We were lucky to get back by him."
Kreitz would retake the lead for good on lap 39, passing Montieth to the low side as they entered Turn 3.
Dietrich moved into third on lap 40 and then took second from Montieth on the following circuit.
T.J. Stutts spun in Turn 4 on lap 45 to bring out the fourth and final caution.
Kreitz would pulled away on the final restart while Montieth closed in on Dietrich for second, but neither drive could catch the veteran at the checkers.
Dietrich would hold off Montieth for second while Smith fell back to fourth and Lance Dewease finished fifth.
Sixth through 10th were Cap Henry, Tim Shaffer, Greg Hodnett, Mike Wagner and Danny Holtgraver, respectively.
Heats for the 40 410 sprint cars were won by Adam Wilt, Stevie Smith, Dewease and Wagner. Brian Leppo won the B Main while Shaffer set fast time with a lap of 16.827 seconds.
Devin Hart won the 12-lap pro stock feature.