The Hemi was only banned for one year in 1965. With the loss of so many top drivers that drove the Hemis because they walked on Nascar because of the ban, Nascar had no choice but to allow the Hemi back in 1966, much to the ire of Ford, who boycotted the first few races of the 1966 campaign.
In 1969, the Dodge Charger Daytona was released, Richard Petty liked the Daytona so much he wanted Plymouth to have the same car, however Plymouth (Plymouth and Dodge racing programs were 2 different entities then), said they weren't going to do that, Richard got mad and drove a Ford during the 1969 racing year. Now comes 1970, and Plymouth releases the Superbird, which brings Richard back to the Plymouth fold driving the famous Petty Blue Bird.
1971 the Superbird was banned (as well as the Daytona) for a couple of reasons. One being that Plymouth made the Superbird with the express purpose of racing in Nascar, making only enough cars for the street to make it legal for Nascar (500 cars), 2nd was the fact that it went so fast, that NASCAR was concerned for the safety of the cars and drivers.
There were in fact about 1900 Superbirds produced (apprx. 135 426 Hemis, 716 440 Six Packs, with the rest being 440 4barrel), and 503 Daytonas, of which 70 had the 426 Hemi.