After the Mustang takes off in the drag race, the boys' car is seen at the Stop sign with several cars behind them. In the next shot looking through the car windshield and every shot after that, no cars are seen behind them, with no evidence of any cars having gone around them.
The wheels on the 1971 Mustang are not stock or from the time period. They are from the 90's.
As they are about to drag race Kevin's narration says the 1965 Oldsmobile f85 they are in has 185 horsepower. Olds offer no engine with that HP output for the 1965 f85. The two engines nearest would have been 155hp and 230hp.
On their Friday night joy ride, the boys encounter older guys who moon them and later want to drag race. The older guys are driving a 1971 Ford Mustang Mach 1. The episode took place in 1970.