There were LOTS of '61, '62, & '63 Big Pontiacs, running on tracks all over the country.
The 389SD & 421SD engines were VERY competitive. So, these cars got lots of class wins & set lots of nat records.
Then, after '63, there was very little factory help. So, MOST of the heavy hitters switched brands. Mopar went "whole hog" into drag racing, giving LOTS of factory support to lots of racers. That, along with the 426 Hemi engines, gave the factory Mopar cars a big advantage. A quick check of race results beginning in the mid '60's will show how dominant Mopar was.
Same thing happened in NASCAR. The dominant 389SD & 421SD Pontiacs gave way to the Hemi Mopars & 427 Fords.
But, in the early '60's, Pontiacs did a lot of winning, on drag & circle tracks. And I love to read about what many call Pontiac's "Glory Days".
There were some crazy classes in '61-'63. NHRA made classes for cars running some factory over-the-counter HP parts, & for very limited production models.
There were classes like S/S, S/SA, OS/S, SS/S, A/FX, B/FX, etc. For a time, those classes, plus A/S were called Top Stock, and classes below that were called Junior Stock.
One of my favorite '61 Pontiacs was raced by div 4 racers, the Moore family, with Gary Moore driving. He set a national record, won class at a nat event, & was div 4 champ, with a '61 Pontiac.
There were lots of dealer sponsored '61's. Another of my favorite '61's was driven by Carol Cox. She won the S/SA class at Pomona & Indy with it. The hottest '61's ran one of the SD engines.