At this point I'm unaware of any official rules created to give less competitive makes an advantage.
The Olds 88 Club Coupe weighed in at 3600+ lbs.
The little Plymouth weighed 2800-2900 lbs.
Olds produced 135 hp stock (27 lbs per hp)
Plymouth produced 97 (around 29 lb per hp)
Old produced 263 ft-lbs stock (14 lb per ft-lb)
Plymouth 175 ft-lbs (16 lb per ft-lb)
So all-in-all, the Plymouth's weren't that far off...I think the real competitive advantage came due to the fact the cars were lighter, therefore easier on tires (this was back in the day of street-stock bias plys rather than hi-tech slicks).
MoPars seemed to do well on tougher handling tracks, where the big Olds, Caddilacs, and Mercurys chewed up tires. Johnny Mantz's Southern 500 winning Plymouth used truck tires that lasted the entire race...3rd place finisher Red Byron's Cadillac went thru no less than 24 tires during the race.
On the longer courses...especially Daytona's high-speed course, the Plymouths could finish no better than mid-pack.
