I think that they always ran 292's. The Pocket Rocket car in the video was their 3rd and last Chevy 11 that they raced(most famous also). Back in the early 70's when Cotton and Jim partnered up, Cotton was a route salseman(truck driver) for a local bread company, and he and Jim were able to get the engines and rear ends out of the trucks that they took out of service(292 engines and Dana 60 rear ends). So I think their choice for the 292's was a convenience issue more than anything.
