I spin my stock bottom 383 to 6400. no issues. 383's are pretty stout. remember that they have a forged crank and the same webbing / oiling / architecture as 440's and Hemi's, only with a smaller 3.38" stroke that does not tax this hardware nearly as hard as the RB's 3.75 stroke. I've always felt that the 383 was about as durable a workhorse as ever came out of an American auto maker.