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No a 400 piston would be a 440 piston. I'm not sure but maybe a 413 piston. 383 rb only has a 4.03 bore. The 383 rb was a stop gap motor, mopar could not provide enough 383 b engine's so the quick fix was the rb.I dought there was a different block than a 413 but you never know. 4.21 is the bore size of the 383 b motor so dont confuse the two yours has a small bore long stroke.
