The hit was so severe it dislodged Ethan Rees’ fuel tank.
The Giants landed big hits on everyone other than the leader over the remainder of the race but Stratford still crossed the line first.
Prentice was second with Peter Rees third, so the result was close, but in the Scrappers' favour, and it was extended when second-placed Prentice was disqualified.
The Giants then accounted for the Great Britain Lions 110-85 on points in race two Friday night, but it meant an uphill battle for them to get a shot at a podium on Saturday night.
They beat the Whanganui Warriors 125 to 70 in the first encounter that night, but it was not enough to go into a run-off for third and fourth.
Instead, the Giants were to meet the Auckland Allstars in the fifth and sixth run-off.
But Auckland declined to race and the Giants were handed fifth place.
Gisborne Speedway president Sam Hughes said: “It was a great spectacle to watch, but you need a bit of luck in speedway, and the Giants just didn’t have that bit of luck at the weekend. .
“As a club, we are still very proud of how they went.
“Now we look forward to the other superstock teams event in Auckland in April.”