Before we leave the SD455 cars, I'll mention that there have been at least one, for sure(don't know how many others), '73 GP that ran an SD455.
I realize that most say that engine only came in Birds. BUT, others have posted that they know of a GP that actually came from the factory with the SD455 engine in it. Don't know if it actually happened or not.
NHRA allows it in the the GP, Lemans, GTO, & GA.
http://www.classracerinfo.com/Engine...px?ENGINE=2080
John Clegg ran a GP with the SD455, one year at Indy. It was the feature car on the front of the H-O newsletter one month.