The rear quarters on some, not all, of the T/G cars were widened to allow wider rear wheels.
It showed most in the gap between the outer part tail lights and the side of the car. That is why the stock bumper was not wide enough, the spoiler looks to be a Camaro item and they came in an early narrower, and later wider version.
I see there were "Stock Car" races at Matsu Bay Japan opening in 72's but due to the fuel crises then they were a short term thing, closing in '73.
Buying a year or two old but well built race car can be a good way to get into racing.
My Firebird came to NZ in late '71, Ron Grable had raced F5000 and Trans Am (Javelin) and it was a fully paid gig during the US winter. He bought the car from Rick Titus and it was sold to a local NZ racer after a couple of weeks racing.