Years ago in the early 90's I performed the same Nascar trick on a 68' Firebird and a 1980 Pontiac Lemans, both running 455's.

For kicks the gentleman with the Lemans took off the set up and the car dropped in real world ET by almost two tenths. Put it back on and each time the car picked right up the lost two tenths, all within a 20 minute window span.
The beauty was that the car retained its stock flat hood look and would carry the wheels for 2 feet or so. Both these car were at the time high to mid 12 second street legal driven cars.