I think a 305 coming back from 2 laps down to finish 7th qualifies as "running with the big motors" and proves the combination could be competitive, at least on the superspeedways. The reason this combination did not continue is because the aero bodies would "age out" after 1971/2 (three year old bodies were the max at the time, and the '69-'70 aero cars were already at the end of their lifespan) and wasn't really worth the costs associated with continued development of the combination. Most (Mopar) teams had already made the transition to the Charger and Roadrunner, which could still run the Hemi engine that was a proven combination that had already had substantial development done to it, and would be able to run the combination for another three seasons.